Make SemIR::TypeType be an empty FacetType instruction (#7813)

The type `type` is now a `FacetType` inst with no constraints. This
brings the model implemented in the toolchain into better alignment with
the language design. The `SemIR::TypeType` struct remains as a scope for
holding the `TypeInstId`, `ConstantId`, and `TypeId` constants, but is
not an `InstKind` anymore.

The `TypeType` inst looks a lot like singletons, but there are many
`FacetType` insts so it doesn't quite fit that model. So we put it
alongside singletons with a fixed inst id but refer to it as a more
general "builtin" inst that is not a singleton.
`Namespace::PackageInstId` is similar, and we group it with `TypeType`
conceptually as another builtin instruction with a fixed id.

No conversion is needed anymore to use a `type` as a facet, since types
also have a `FacetType` type. This simplifies and removes a number of
helpers and branches throughout the code.

The `TypeType` inst is now part of the constant store, so we end up
printing it in the constants block in every test. But it's also named
`type` rather than `%type` to preserve the majority of existing
formatting behaviour, though this does look different from other
constants.

Assisted-by: Opus 5 was used to generate a first draft and validate the
refactoring. Though nearly everything non-trivial the tool wrote has
been modified or rewritten.

toolchain/check/testdata/interop/cpp/impls/as.carbon

@@ -132,6 +132,7 @@ fn ConversionExplicit(s: Cpp.ConditionallyExplicitFalse) {
 // CHECK:STDOUT: --- conversions.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %Source: type = class_type @Source [concrete]
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %Dest: type = class_type @Dest [concrete]
@@ -301,6 +302,7 @@ fn ConversionExplicit(s: Cpp.ConditionallyExplicitFalse) {
 // CHECK:STDOUT: --- conditionally_explicit.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %Source: type = class_type @Source [concrete]
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %ConditionallyExplicit.3cd: type = class_type @ConditionallyExplicit.1 [concrete]
@@ -502,6 +504,7 @@ fn ConversionExplicit(s: Cpp.ConditionallyExplicitFalse) {
 // CHECK:STDOUT: --- fail_conditionally_explicit_implicit.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %Source: type = class_type @Source [concrete]
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %ConditionallyExplicit.9d4: type = class_type @ConditionallyExplicit.1 [concrete]

toolchain/check/testdata/interop/cpp/impls/copy.carbon

@@ -195,6 +195,7 @@ fn EqualWitnesses(p: Wrap(Cpp.Copyable)*) -> Wrap(Cpp.Copyable)* {
 // CHECK:STDOUT: --- copy_copyable.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %Copyable: type = class_type @Copyable [concrete]
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %Copy.type: type = facet_type <@Copy> [concrete]
@@ -291,6 +292,7 @@ fn EqualWitnesses(p: Wrap(Cpp.Copyable)*) -> Wrap(Cpp.Copyable)* {
 // CHECK:STDOUT: --- copy_generically.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %Copy.type.bd0: type = facet_type <@Copy.1> [concrete]
 // CHECK:STDOUT:   %Copy.type.705: type = fn_type @Copy.loc6 [concrete]
 // CHECK:STDOUT:   %Copy: %Copy.type.705 = struct_value () [concrete]

toolchain/check/testdata/interop/cpp/impls/destroy.carbon

@@ -227,6 +227,7 @@ fn F() {
 // CHECK:STDOUT: --- destroy_destroyable.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %PublicDestroy.type: type = fn_type @PublicDestroy [concrete]
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %PublicDestroy: %PublicDestroy.type = struct_value () [concrete]
@@ -384,6 +385,7 @@ fn F() {
 // CHECK:STDOUT: --- fail_todo_destroy_protected_base_destructor.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %Derived: type = class_type @Derived [concrete]
 // CHECK:STDOUT:   %ProtectedDestructor: type = class_type @ProtectedDestructor [concrete]
 // CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
@@ -439,6 +441,7 @@ fn F() {
 // CHECK:STDOUT: --- fail_destroy_private_base_destructor.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %Derived: type = class_type @Derived [concrete]
 // CHECK:STDOUT:   %PrivateDestructor: type = class_type @PrivateDestructor [concrete]
 // CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]

toolchain/check/testdata/interop/cpp/impls/implicit_as.carbon

@@ -414,6 +414,7 @@ fn InitFromStruct() {
 // CHECK:STDOUT: --- implicit_conversions.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %Source: type = class_type @Source [concrete]
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %Dest: type = class_type @Dest [concrete]
@@ -566,6 +567,7 @@ fn InitFromStruct() {
 // CHECK:STDOUT: --- fail_expr_category.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %NonConstConversion: type = class_type @NonConstConversion [concrete]
 // CHECK:STDOUT:   %Dest.980: type = class_type @Dest [concrete]
 // CHECK:STDOUT:   %pattern_type.b72: type = pattern_type %Dest.980 [concrete]
@@ -600,6 +602,7 @@ fn InitFromStruct() {
 // CHECK:STDOUT: --- fail_access.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %Source: type = class_type @Source [concrete]
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %InaccessibleConstructor: type = class_type @InaccessibleConstructor [concrete]
@@ -717,6 +720,7 @@ fn InitFromStruct() {
 // CHECK:STDOUT: --- fail_deleted.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %Source: type = class_type @Source [concrete]
 // CHECK:STDOUT:   %DeletedConstructor: type = class_type @DeletedConstructor [concrete]
 // CHECK:STDOUT:   %pattern_type.937: type = pattern_type %DeletedConstructor [concrete]
@@ -774,6 +778,7 @@ fn InitFromStruct() {
 // CHECK:STDOUT: --- fail_explicit.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %Source: type = class_type @Source [concrete]
 // CHECK:STDOUT:   %ExplicitConstructor: type = class_type @ExplicitConstructor [concrete]
 // CHECK:STDOUT:   %pattern_type.eae: type = pattern_type %ExplicitConstructor [concrete]
@@ -831,6 +836,7 @@ fn InitFromStruct() {
 // CHECK:STDOUT: --- i32_to_int.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %int_32: Core.IntLiteral = int_value 32 [concrete]
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %i32: type = class_type @Int, @Int(%int_32) [concrete]
@@ -893,6 +899,7 @@ fn InitFromStruct() {
 // CHECK:STDOUT: --- fail_no_u32_to_int_conversion_in_carbon.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %int_32: Core.IntLiteral = int_value 32 [concrete]
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %u32: type = class_type @UInt, @UInt(%int_32) [concrete]
@@ -957,6 +964,7 @@ fn InitFromStruct() {
 // CHECK:STDOUT: --- default_constructor.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %DefaultConstructor: type = class_type @DefaultConstructor [concrete]
 // CHECK:STDOUT:   %pattern_type.be4: type = pattern_type %DefaultConstructor [concrete]
@@ -1018,6 +1026,7 @@ fn InitFromStruct() {
 // CHECK:STDOUT: --- construct_multi_argument.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %Two: type = class_type @Two [concrete]
 // CHECK:STDOUT:   %pattern_type.586: type = pattern_type %Two [concrete]
@@ -1235,6 +1244,7 @@ fn InitFromStruct() {
 // CHECK:STDOUT: --- fail_todo_aggregate_from_tuple.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %Aggregate: type = class_type @Aggregate [concrete]
 // CHECK:STDOUT:   %pattern_type.5dc: type = pattern_type %Aggregate [concrete]
@@ -1297,6 +1307,7 @@ fn InitFromStruct() {
 // CHECK:STDOUT: --- fail_todo_aggregate_from_struct.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %Aggregate: type = class_type @Aggregate [concrete]
 // CHECK:STDOUT:   %pattern_type.5dc: type = pattern_type %Aggregate [concrete]
 // CHECK:STDOUT:   %_.patt: %pattern_type.5dc = value_binding_pattern _ [concrete]
@@ -1359,6 +1370,7 @@ fn InitFromStruct() {
 // CHECK:STDOUT: --- non_aggregate_from_tuple.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %NonAggregate: type = class_type @NonAggregate [concrete]
 // CHECK:STDOUT:   %pattern_type.273: type = pattern_type %NonAggregate [concrete]
@@ -1525,6 +1537,7 @@ fn InitFromStruct() {
 // CHECK:STDOUT: --- fail_non_aggregate_from_struct.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
+// CHECK:STDOUT:   type: type = facet_type <type> [concrete]
 // CHECK:STDOUT:   %NonAggregate: type = class_type @NonAggregate [concrete]
 // CHECK:STDOUT:   %pattern_type.273: type = pattern_type %NonAggregate [concrete]
 // CHECK:STDOUT:   %_.patt: %pattern_type.273 = value_binding_pattern _ [concrete]