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/interop/cpp/class/abstract.carbon

@@ -94,6 +94,7 @@ fn F() -> Cpp.AbstractFinal {
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %AbstractFinal: type = class_type @AbstractFinal [concrete]
+// CHECK:STDOUT:   %.fd6: form = init_form %AbstractFinal, call_param0 [concrete]
 // CHECK:STDOUT:   %pattern_type: type = pattern_type %AbstractFinal [concrete]
 // CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
 // CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
@@ -114,6 +115,7 @@ fn F() -> Cpp.AbstractFinal {
 // CHECK:STDOUT:   } {
 // CHECK:STDOUT:     %Cpp.ref: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:     %AbstractFinal.ref: type = name_ref AbstractFinal, imports.%AbstractFinal.decl [concrete = constants.%AbstractFinal]
+// CHECK:STDOUT:     %.loc11_14: form = init_form %AbstractFinal.ref, call_param0 [concrete = constants.%.fd6]
 // CHECK:STDOUT:     %return.param: ref %AbstractFinal = out_param call_param0
 // CHECK:STDOUT:     %return: ref %AbstractFinal = return_slot %return.param
 // CHECK:STDOUT:   }
@@ -122,8 +124,8 @@ fn F() -> Cpp.AbstractFinal {
 // CHECK:STDOUT: fn @F() -> %return.param: %AbstractFinal {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %F.ref: %F.type = name_ref F, file.%F.decl [concrete = constants.%F]
-// CHECK:STDOUT:   %.loc11: ref %AbstractFinal = splice_block %return {}
-// CHECK:STDOUT:   %F.call: init %AbstractFinal = call %F.ref() to %.loc11
+// CHECK:STDOUT:   %.loc11_8: ref %AbstractFinal = splice_block %return {}
+// CHECK:STDOUT:   %F.call: init %AbstractFinal = call %F.ref() to %.loc11_8
 // CHECK:STDOUT:   return %F.call to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:

toolchain/check/testdata/interop/cpp/class/base.carbon

@@ -465,6 +465,7 @@ class V {
 // CHECK:STDOUT:   %pattern_type.5c86: type = pattern_type %ptr.ddb [concrete]
 // CHECK:STDOUT:   %Base: type = class_type @Base [concrete]
 // CHECK:STDOUT:   %ptr.fb2: type = ptr_type %Base [concrete]
+// CHECK:STDOUT:   %.43a: form = init_form %ptr.fb2, call_param1 [concrete]
 // CHECK:STDOUT:   %pattern_type.72a: type = pattern_type %ptr.fb2 [concrete]
 // CHECK:STDOUT:   %ConvertPtr.type: type = fn_type @ConvertPtr [concrete]
 // CHECK:STDOUT:   %ConvertPtr: %ConvertPtr.type = struct_value () [concrete]
@@ -508,8 +509,9 @@ class V {
 // CHECK:STDOUT:     %Cpp.ref.loc7_35: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:     %Base.ref: type = name_ref Base, imports.%Base.decl [concrete = constants.%Base]
 // CHECK:STDOUT:     %ptr.loc7_43: type = ptr_type %Base.ref [concrete = constants.%ptr.fb2]
+// CHECK:STDOUT:     %.loc7_43: form = init_form %ptr.loc7_43, call_param1 [concrete = constants.%.43a]
 // CHECK:STDOUT:     %d.param: %ptr.ddb = value_param call_param0
-// CHECK:STDOUT:     %.loc7: type = splice_block %ptr.loc7_29 [concrete = constants.%ptr.ddb] {
+// CHECK:STDOUT:     %.loc7_29: type = splice_block %ptr.loc7_29 [concrete = constants.%ptr.ddb] {
 // CHECK:STDOUT:       %Cpp.ref.loc7_18: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:       %Derived.ref: type = name_ref Derived, imports.%Derived.decl [concrete = constants.%Derived]
 // CHECK:STDOUT:       %ptr.loc7_29: type = ptr_type %Derived.ref [concrete = constants.%ptr.ddb]

toolchain/check/testdata/interop/cpp/stdlib/string_view.carbon

@@ -75,6 +75,7 @@ fn G() -> str {
 // CHECK:STDOUT:   %ptr.85f: type = ptr_type %str.ee0 [concrete]
 // CHECK:STDOUT:   %Consume__carbon_thunk.type: type = fn_type @Consume__carbon_thunk [concrete]
 // CHECK:STDOUT:   %Consume__carbon_thunk: %Consume__carbon_thunk.type = struct_value () [concrete]
+// CHECK:STDOUT:   %.a70: form = init_form %str.ee0, call_param0 [concrete]
 // CHECK:STDOUT:   %G.type: type = fn_type @G [concrete]
 // CHECK:STDOUT:   %G: %G.type = struct_value () [concrete]
 // CHECK:STDOUT:   %Produce.cpp_overload_set.type: type = cpp_overload_set_type @Produce.cpp_overload_set [concrete]
@@ -109,6 +110,7 @@ fn G() -> str {
 // CHECK:STDOUT:     %return.patt: %pattern_type.461 = return_slot_pattern [concrete]
 // CHECK:STDOUT:     %return.param_patt: %pattern_type.461 = out_param_pattern %return.patt, call_param0 [concrete]
 // CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %.loc13_11: form = init_form constants.%str.ee0, call_param0 [concrete = constants.%.a70]
 // CHECK:STDOUT:     %return.param: ref %str.ee0 = out_param call_param0
 // CHECK:STDOUT:     %return: ref %str.ee0 = return_slot %return.param
 // CHECK:STDOUT:   }
@@ -131,10 +133,10 @@ fn G() -> str {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %Cpp.ref: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:   %Produce.ref: %Produce.cpp_overload_set.type = name_ref Produce, imports.%Produce.cpp_overload_set.value [concrete = constants.%Produce.cpp_overload_set.value]
-// CHECK:STDOUT:   %.loc13: ref %str.ee0 = splice_block %return {}
-// CHECK:STDOUT:   %addr: %ptr.85f = addr_of %.loc13
+// CHECK:STDOUT:   %.loc13_8: ref %str.ee0 = splice_block %return {}
+// CHECK:STDOUT:   %addr: %ptr.85f = addr_of %.loc13_8
 // CHECK:STDOUT:   %Produce__carbon_thunk.call: init %empty_tuple.type = call imports.%Produce__carbon_thunk.decl(%addr)
-// CHECK:STDOUT:   %.loc14: init %str.ee0 = in_place_init %Produce__carbon_thunk.call, %.loc13
+// CHECK:STDOUT:   %.loc14: init %str.ee0 = in_place_init %Produce__carbon_thunk.call, %.loc13_8
 // CHECK:STDOUT:   return %.loc14 to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT: