Initial support for return forms (#6556)

The main changes here are:
- Introducing `InitForm` and `RefForm` to represent initializing and
reference forms (the two return forms currently supported by the
parser).
- Introducing the `FormType` singleton inst to represent their type
(i.e. `Core.Form`).
- Emitting an inst representing a function's declared return form as
part of handling the function signature.

The return form inst is currently ignored. Subsequent PRs will expose it
in `SemIR::Function` and use it to determine the form of call
expressions.

toolchain/check/testdata/pointer/address_of_deref.carbon

@@ -26,6 +26,7 @@ fn F() -> i32 {
 // CHECK:STDOUT:   %Int.generic: %Int.type = struct_value () [concrete]
 // CHECK:STDOUT:   %N: Core.IntLiteral = symbolic_binding N, 0 [symbolic]
 // CHECK:STDOUT:   %i32: type = class_type @Int, @Int(%int_32) [concrete]
+// CHECK:STDOUT:   %.941: form = init_form %i32, call_param0 [concrete]
 // CHECK:STDOUT:   %pattern_type.7ce: type = pattern_type %i32 [concrete]
 // CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
 // CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
@@ -93,6 +94,7 @@ fn F() -> i32 {
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     %int_32.loc15: Core.IntLiteral = int_value 32 [concrete = constants.%int_32]
 // CHECK:STDOUT:     %i32.loc15: type = class_type @Int, @Int(constants.%int_32) [concrete = constants.%i32]
+// CHECK:STDOUT:     %.loc15: form = init_form %i32.loc15, call_param0 [concrete = constants.%.941]
 // CHECK:STDOUT:     %return.param: ref %i32 = out_param call_param0
 // CHECK:STDOUT:     %return: ref %i32 = return_slot %return.param
 // CHECK:STDOUT:   }

toolchain/check/testdata/pointer/basic.carbon

@@ -28,6 +28,7 @@ fn F() -> i32 {
 // CHECK:STDOUT:   %Int.generic: %Int.type = struct_value () [concrete]
 // CHECK:STDOUT:   %N: Core.IntLiteral = symbolic_binding N, 0 [symbolic]
 // CHECK:STDOUT:   %i32: type = class_type @Int, @Int(%int_32) [concrete]
+// CHECK:STDOUT:   %.941: form = init_form %i32, call_param0 [concrete]
 // CHECK:STDOUT:   %pattern_type.7ce: type = pattern_type %i32 [concrete]
 // CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
 // CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
@@ -109,6 +110,7 @@ fn F() -> i32 {
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     %int_32.loc15: Core.IntLiteral = int_value 32 [concrete = constants.%int_32]
 // CHECK:STDOUT:     %i32.loc15: type = class_type @Int, @Int(constants.%int_32) [concrete = constants.%i32]
+// CHECK:STDOUT:     %.loc15: form = init_form %i32.loc15, call_param0 [concrete = constants.%.941]
 // CHECK:STDOUT:     %return.param: ref %i32 = out_param call_param0
 // CHECK:STDOUT:     %return: ref %i32 = return_slot %return.param
 // CHECK:STDOUT:   }

toolchain/check/testdata/pointer/fail_type_mismatch.carbon

@@ -33,6 +33,7 @@ fn ConstMismatch(p: const {}*) -> const ({}*) {
 // CHECK:STDOUT:   %pattern_type.db0: type = pattern_type %ptr.bf9 [concrete]
 // CHECK:STDOUT:   %ptr.c28: type = ptr_type %empty_struct_type [concrete]
 // CHECK:STDOUT:   %const.987: type = const_type %ptr.c28 [concrete]
+// CHECK:STDOUT:   %.351: form = init_form %const.987, call_param1 [concrete]
 // CHECK:STDOUT:   %pattern_type.3c6: type = pattern_type %const.987 [concrete]
 // CHECK:STDOUT:   %ConstMismatch.type: type = fn_type @ConstMismatch [concrete]
 // CHECK:STDOUT:   %ConstMismatch: %ConstMismatch.type = struct_value () [concrete]
@@ -65,6 +66,7 @@ fn ConstMismatch(p: const {}*) -> const ({}*) {
 // CHECK:STDOUT:     %.loc15_44: type = converted %.loc15_43, constants.%empty_struct_type [concrete = constants.%empty_struct_type]
 // CHECK:STDOUT:     %ptr.loc15_44: type = ptr_type %.loc15_44 [concrete = constants.%ptr.c28]
 // CHECK:STDOUT:     %const.loc15_35: type = const_type %ptr.loc15_44 [concrete = constants.%const.987]
+// CHECK:STDOUT:     %.loc15_35: form = init_form %const.loc15_35, call_param1 [concrete = constants.%.351]
 // CHECK:STDOUT:     %p.param: %ptr.bf9 = value_param call_param0
 // CHECK:STDOUT:     %.loc15_29: type = splice_block %ptr.loc15_29 [concrete = constants.%ptr.bf9] {
 // CHECK:STDOUT:       %.loc15_28: %empty_struct_type = struct_literal () [concrete = constants.%empty_struct]

toolchain/check/testdata/pointer/nested_const.carbon

@@ -31,6 +31,7 @@ fn F(p: const (const (const i32*)*)) -> const i32 {
 // CHECK:STDOUT:   %ptr.6e8: type = ptr_type %const.58f [concrete]
 // CHECK:STDOUT:   %const.fa2: type = const_type %ptr.6e8 [concrete]
 // CHECK:STDOUT:   %pattern_type.800: type = pattern_type %const.fa2 [concrete]
+// CHECK:STDOUT:   %.d3c: form = init_form %const.20a, call_param1 [concrete]
 // CHECK:STDOUT:   %pattern_type.a65: type = pattern_type %const.20a [concrete]
 // CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
 // CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
@@ -81,8 +82,9 @@ fn F(p: const (const (const i32*)*)) -> const i32 {
 // CHECK:STDOUT:     %int_32.loc16_47: Core.IntLiteral = int_value 32 [concrete = constants.%int_32]
 // CHECK:STDOUT:     %i32.loc16_47: type = class_type @Int, @Int(constants.%int_32) [concrete = constants.%i32]
 // CHECK:STDOUT:     %const.loc16_41: type = const_type %i32.loc16_47 [concrete = constants.%const.20a]
+// CHECK:STDOUT:     %.loc16_41: form = init_form %const.loc16_41, call_param1 [concrete = constants.%.d3c]
 // CHECK:STDOUT:     %p.param: %const.fa2 = value_param call_param0
-// CHECK:STDOUT:     %.loc16: type = splice_block %const.loc16_9 [concrete = constants.%const.fa2] {
+// CHECK:STDOUT:     %.loc16_9: type = splice_block %const.loc16_9 [concrete = constants.%const.fa2] {
 // CHECK:STDOUT:       %int_32.loc16_29: Core.IntLiteral = int_value 32 [concrete = constants.%int_32]
 // CHECK:STDOUT:       %i32.loc16_29: type = class_type @Int, @Int(constants.%int_32) [concrete = constants.%i32]
 // CHECK:STDOUT:       %const.loc16_23: type = const_type %i32.loc16_29 [concrete = constants.%const.20a]

toolchain/check/testdata/pointer/types.carbon

@@ -29,6 +29,7 @@ fn ConstPtr(p: const i32*) -> (const i32)* {
 // CHECK:STDOUT:   %i32: type = class_type @Int, @Int(%int_32) [concrete]
 // CHECK:STDOUT:   %ptr.235: type = ptr_type %i32 [concrete]
 // CHECK:STDOUT:   %pattern_type.fe8: type = pattern_type %ptr.235 [concrete]
+// CHECK:STDOUT:   %.4f9: form = init_form %ptr.235, call_param1 [concrete]
 // CHECK:STDOUT:   %Ptr.type: type = fn_type @Ptr [concrete]
 // CHECK:STDOUT:   %Ptr: %Ptr.type = struct_value () [concrete]
 // CHECK:STDOUT:   %Copy.type: type = facet_type <@Copy> [concrete]
@@ -45,6 +46,7 @@ fn ConstPtr(p: const i32*) -> (const i32)* {
 // CHECK:STDOUT:   %const.20a: type = const_type %i32 [concrete]
 // CHECK:STDOUT:   %ptr.36b: type = ptr_type %const.20a [concrete]
 // CHECK:STDOUT:   %pattern_type.bff: type = pattern_type %ptr.36b [concrete]
+// CHECK:STDOUT:   %.e29: form = init_form %ptr.36b, call_param1 [concrete]
 // CHECK:STDOUT:   %ConstPtr.type: type = fn_type @ConstPtr [concrete]
 // CHECK:STDOUT:   %ConstPtr: %ConstPtr.type = struct_value () [concrete]
 // CHECK:STDOUT:   %Copy.impl_witness.ce1: <witness> = impl_witness imports.%Copy.impl_witness_table.c3a, @ptr.as.Copy.impl(%const.20a) [concrete]
@@ -84,8 +86,9 @@ fn ConstPtr(p: const i32*) -> (const i32)* {
 // CHECK:STDOUT:     %int_32.loc15_20: Core.IntLiteral = int_value 32 [concrete = constants.%int_32]
 // CHECK:STDOUT:     %i32.loc15_20: type = class_type @Int, @Int(constants.%int_32) [concrete = constants.%i32]
 // CHECK:STDOUT:     %ptr.loc15_23: type = ptr_type %i32.loc15_20 [concrete = constants.%ptr.235]
+// CHECK:STDOUT:     %.loc15_23: form = init_form %ptr.loc15_23, call_param1 [concrete = constants.%.4f9]
 // CHECK:STDOUT:     %p.param: %ptr.235 = value_param call_param0
-// CHECK:STDOUT:     %.loc15: type = splice_block %ptr.loc15_14 [concrete = constants.%ptr.235] {
+// CHECK:STDOUT:     %.loc15_14: type = splice_block %ptr.loc15_14 [concrete = constants.%ptr.235] {
 // CHECK:STDOUT:       %int_32.loc15_11: Core.IntLiteral = int_value 32 [concrete = constants.%int_32]
 // CHECK:STDOUT:       %i32.loc15_11: type = class_type @Int, @Int(constants.%int_32) [concrete = constants.%i32]
 // CHECK:STDOUT:       %ptr.loc15_14: type = ptr_type %i32.loc15_11 [concrete = constants.%ptr.235]
@@ -104,8 +107,9 @@ fn ConstPtr(p: const i32*) -> (const i32)* {
 // CHECK:STDOUT:     %i32.loc19_38: type = class_type @Int, @Int(constants.%int_32) [concrete = constants.%i32]
 // CHECK:STDOUT:     %const.loc19_32: type = const_type %i32.loc19_38 [concrete = constants.%const.20a]
 // CHECK:STDOUT:     %ptr.loc19_42: type = ptr_type %const.loc19_32 [concrete = constants.%ptr.36b]
+// CHECK:STDOUT:     %.loc19_42: form = init_form %ptr.loc19_42, call_param1 [concrete = constants.%.e29]
 // CHECK:STDOUT:     %p.param: %ptr.36b = value_param call_param0
-// CHECK:STDOUT:     %.loc19: type = splice_block %ptr.loc19_25 [concrete = constants.%ptr.36b] {
+// CHECK:STDOUT:     %.loc19_25: type = splice_block %ptr.loc19_25 [concrete = constants.%ptr.36b] {
 // CHECK:STDOUT:       %int_32.loc19_22: Core.IntLiteral = int_value 32 [concrete = constants.%int_32]
 // CHECK:STDOUT:       %i32.loc19_22: type = class_type @Int, @Int(constants.%int_32) [concrete = constants.%i32]
 // CHECK:STDOUT:       %const.loc19_16: type = const_type %i32.loc19_22 [concrete = constants.%const.20a]