Add unit tagging to StructTypeFieldId (#6260)

Based on #6259

toolchain/check/testdata/basics/raw_sem_ir/cpp_interop.carbon

@@ -74,9 +74,9 @@ fn G(x: Cpp.X) {
 // CHECK:STDOUT:   specifics:       {}
 // CHECK:STDOUT:   struct_type_fields:
 // CHECK:STDOUT:     struct_type_fields00000000: {}
-// CHECK:STDOUT:     struct_type_fields00000001:
+// CHECK:STDOUT:     struct_type_fields60000001:
 // CHECK:STDOUT:       0:               {name_id: name00000006, type_inst_id: inst6000001F}
-// CHECK:STDOUT:     struct_type_fields00000002:
+// CHECK:STDOUT:     struct_type_fields60000002:
 // CHECK:STDOUT:       0:               {name_id: name00000006, type_inst_id: inst6000001F}
 // CHECK:STDOUT:   types:
 // CHECK:STDOUT:     'type(TypeType)':
@@ -144,8 +144,8 @@ fn G(x: Cpp.X) {
 // CHECK:STDOUT:     inst6000001F:    {kind: PointerType, arg0: inst60000013, type: type(TypeType)}
 // CHECK:STDOUT:     inst60000020:    {kind: UnboundElementType, arg0: inst60000013, arg1: inst6000001F, type: type(TypeType)}
 // CHECK:STDOUT:     inst60000021:    {kind: FieldDecl, arg0: name00000006, arg1: element0, type: type(inst60000020)}
-// CHECK:STDOUT:     inst60000022:    {kind: CustomLayoutType, arg0: struct_type_fields00000001, arg1: custom_layout00000001, type: type(TypeType)}
-// CHECK:STDOUT:     inst60000023:    {kind: CustomLayoutType, arg0: struct_type_fields00000002, arg1: custom_layout00000001, type: type(TypeType)}
+// CHECK:STDOUT:     inst60000022:    {kind: CustomLayoutType, arg0: struct_type_fields60000001, arg1: custom_layout00000001, type: type(TypeType)}
+// CHECK:STDOUT:     inst60000023:    {kind: CustomLayoutType, arg0: struct_type_fields60000002, arg1: custom_layout00000001, type: type(TypeType)}
 // CHECK:STDOUT:     inst60000024:    {kind: CompleteTypeWitness, arg0: inst60000022, type: type(inst(WitnessType))}
 // CHECK:STDOUT:     inst60000025:    {kind: CompleteTypeWitness, arg0: inst60000023, type: type(inst(WitnessType))}
 // CHECK:STDOUT:     inst60000026:    {kind: PointerType, arg0: inst60000023, type: type(TypeType)}

toolchain/sem_ir/file.cpp

@@ -55,7 +55,9 @@ File::File(const Parse::Tree* parse_tree, CheckIRId check_ir_id,
       vtables_(check_ir_id),
       constant_values_(ConstantId::NotConstant, &insts_),
       inst_blocks_(allocator_, check_ir_id),
-      constants_(this) {
+      constants_(this),
+      // 1 reserved id for `StructTypeFields::Empty`.
+      struct_type_fields_(allocator_, IdTag(check_ir_id.index, 1)) {
   // `type` and the error type are both complete & concrete types.
   types_.SetComplete(
       TypeType::TypeId,

toolchain/sem_ir/file.h

@@ -403,7 +403,7 @@ class File : public Printable<File> {
   ConstantStore constants_;
 
   // Storage for StructTypeField lists.
-  StructTypeFieldsStore struct_type_fields_ = StructTypeFieldsStore(allocator_);
+  StructTypeFieldsStore struct_type_fields_;
 
   // Storage for custom layouts.
   CustomLayoutStore custom_layouts_ = CustomLayoutStore(allocator_);