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_function.cpp

@@ -39,8 +39,8 @@ auto HandleReturnType(Context& context, Parse::ReturnTypeId node_id) -> bool {
   auto [type_node_id, type_inst_id] = context.node_stack().PopExprWithNodeId();
   auto type_id = ExprAsType(context, type_node_id, type_inst_id);
   // TODO: Use a dedicated instruction rather than VarStorage here.
-  context.AddInstAndPush(
-      {node_id, SemIR::VarStorage{type_id, SemIR::NameId::ReturnSlot}});
+  context.AddInstAndPush<SemIR::VarStorage>(
+      node_id, {.type_id = type_id, .name_id = SemIR::NameId::ReturnSlot});
   return true;
 }
 
@@ -254,7 +254,8 @@ static auto BuildFunctionDecl(Context& context,
   auto function_info = SemIR::Function{
       .name_id = name_context.name_id_for_new_inst(),
       .enclosing_scope_id = name_context.enclosing_scope_id_for_new_inst(),
-      .decl_id = context.AddPlaceholderInst({node_id, function_decl}),
+      .decl_id = context.AddPlaceholderInst(
+          SemIR::LocIdAndInst(node_id, function_decl)),
       .implicit_param_refs_id = name.implicit_params_id,
       .param_refs_id = name.params_id,
       .return_type_id = return_type_id,
@@ -406,7 +407,7 @@ auto HandleFunctionDefinition(Context& context,
           "Missing `return` at end of function with declared return type.");
       context.emitter().Emit(TokenOnly(node_id), MissingReturnStatement);
     } else {
-      context.AddInst({node_id, SemIR::Return{}});
+      context.AddInst<SemIR::Return>(node_id, {});
     }
   }