Switch AddInst struct init style. (#4012)

Trying to conform with #4009. Changes SemIR::LocIdAndInst construction
to root out struct init cases with AddInst and related functions. I'm
using templating of AddInst functions in order to avoid `AddInst(loc_id,
InstName{...})` and instead have `AddInst<InstName>(loc_id, {...})` with
I think similar readability results. There are a couple cases where inst
construction is templated and so designated initializers couldn't be
used, so this may be better for those in particular due to the extra
type enforcement.

This probably doesn't clean up every last case, but I was trying to get
the bulk at once without bleeding over into less related changes.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/handle_class.cpp

@@ -11,6 +11,7 @@
 #include "toolchain/check/modifiers.h"
 #include "toolchain/check/name_component.h"
 #include "toolchain/sem_ir/ids.h"
+#include "toolchain/sem_ir/inst.h"
 #include "toolchain/sem_ir/typed_insts.h"
 
 namespace Carbon::Check {
@@ -211,9 +212,11 @@ static auto BuildClassDecl(Context& context, Parse::AnyClassDeclId node_id,
   auto decl_block_id = context.inst_block_stack().Pop();
 
   // Add the class declaration.
-  auto class_decl = SemIR::ClassDecl{SemIR::TypeId::TypeType,
-                                     SemIR::ClassId::Invalid, decl_block_id};
-  auto class_decl_id = context.AddPlaceholderInst({node_id, class_decl});
+  auto class_decl = SemIR::ClassDecl{.type_id = SemIR::TypeId::TypeType,
+                                     .class_id = SemIR::ClassId::Invalid,
+                                     .decl_block_id = decl_block_id};
+  auto class_decl_id =
+      context.AddPlaceholderInst(SemIR::LocIdAndInst(node_id, class_decl));
 
   // TODO: Store state regarding is_extern.
   SemIR::Class class_info = {
@@ -388,8 +391,8 @@ auto HandleAdaptDecl(Context& context, Parse::AdaptDeclId node_id) -> bool {
   });
 
   // Build a SemIR representation for the declaration.
-  class_info.adapt_id =
-      context.AddInst({node_id, SemIR::AdaptDecl{adapted_type_id}});
+  class_info.adapt_id = context.AddInst<SemIR::AdaptDecl>(
+      node_id, {.adapted_type_id = adapted_type_id});
 
   // Extend the class scope with the adapted type's scope if requested.
   if (modifiers.HasAnyOf(KeywordModifierSet::Extend)) {
@@ -520,19 +523,20 @@ auto HandleBaseDecl(Context& context, Parse::BaseDeclId node_id) -> bool {
   // binding will be performed when it's found by name lookup into an instance.
   auto field_type_id =
       context.GetUnboundElementType(class_info.self_type_id, base_info.type_id);
-  class_info.base_id = context.AddInst(
-      {node_id,
-       SemIR::BaseDecl{field_type_id, base_info.type_id,
-                       SemIR::ElementIndex(context.args_type_info_stack()
-                                               .PeekCurrentBlockContents()
-                                               .size())}});
+  class_info.base_id = context.AddInst<SemIR::BaseDecl>(
+      node_id,
+      {.type_id = field_type_id,
+       .base_type_id = base_info.type_id,
+       .index = SemIR::ElementIndex(
+           context.args_type_info_stack().PeekCurrentBlockContents().size())});
 
   // Add a corresponding field to the object representation of the class.
   // TODO: Consider whether we want to use `partial T` here.
   // TODO: Should we diagnose if there are already any fields?
-  context.args_type_info_stack().AddInstId(context.AddInstInNoBlock(
-      {node_id,
-       SemIR::StructTypeField{SemIR::NameId::Base, base_info.type_id}}));
+  context.args_type_info_stack().AddInstId(
+      context.AddInstInNoBlock<SemIR::StructTypeField>(
+          node_id, {.name_id = SemIR::NameId::Base,
+                    .field_type_id = base_info.type_id}));
 
   // Bind the name `base` in the class to the base field.
   context.decl_name_stack().AddNameOrDiagnoseDuplicate(