Change struct literal parsing to use placeholders. (#3850)

This is achieving a similar goal as #3849, using placeholders instead of
an ambiguous start node to clarify structure and incrementally simplify
checking. The benefit isn't quite as big here because both paths are
structs, and so checking is more consistent than paren exprs versus
tuples. But I think this removes the only other multi-purpose parse
node.

This uses StructLiteral/StructTypeLiteral naming, reflecting equivalent
SemIR naming. Note, I would lean towards renaming StructLiteral to
StructValueLiteral, but I think consistency in naming takes precedence.
Any renames of StructLiteral might be better in a separate PR.

StructFieldType/StructFieldValue -> StructTypeField/StructField is
trying to making the reading more consistent with
StructTypeLiteral/StructLiteral. SemIR has StructTypeField but not a
value equivalent.

toolchain/check/handle_struct.cpp

@@ -7,14 +7,19 @@
 
 namespace Carbon::Check {
 
-auto HandleStructLiteralOrStructTypeLiteralStart(
-    Context& context, Parse::StructLiteralOrStructTypeLiteralStartId node_id)
+auto HandleStructTypeLiteralStart(Context& context,
+                                  Parse::StructTypeLiteralStartId node_id)
     -> bool {
   context.scope_stack().Push();
   context.node_stack().Push(node_id);
-  // At this point we aren't sure whether this will be a value or type literal,
-  // so we push onto args irrespective. It just won't be used for a type
-  // literal.
+  context.param_and_arg_refs_stack().Push();
+  return true;
+}
+
+auto HandleStructLiteralStart(Context& context,
+                              Parse::StructLiteralStartId node_id) -> bool {
+  context.scope_stack().Push();
+  context.node_stack().Push(node_id);
   context.args_type_info_stack().Push();
   context.param_and_arg_refs_stack().Push();
   return true;
@@ -34,8 +39,7 @@ auto HandleStructComma(Context& context, Parse::StructCommaId /*node_id*/)
   return true;
 }
 
-auto HandleStructFieldValue(Context& context, Parse::StructFieldValueId node_id)
-    -> bool {
+auto HandleStructField(Context& context, Parse::StructFieldId node_id) -> bool {
   auto value_inst_id = context.node_stack().PopExpr();
   auto [name_node, name_id] = context.node_stack().PopNameWithNodeId();
 
@@ -49,7 +53,7 @@ auto HandleStructFieldValue(Context& context, Parse::StructFieldValueId node_id)
   return true;
 }
 
-auto HandleStructFieldType(Context& context, Parse::StructFieldTypeId node_id)
+auto HandleStructTypeField(Context& context, Parse::StructTypeFieldId node_id)
     -> bool {
   auto [type_node, type_id] = context.node_stack().PopExprWithNodeId();
   SemIR::TypeId cast_type_id = ExprAsType(context, type_node, type_id);
@@ -92,12 +96,11 @@ static auto DiagnoseDuplicateNames(Context& context,
 auto HandleStructLiteral(Context& context, Parse::StructLiteralId node_id)
     -> bool {
   auto refs_id = context.param_and_arg_refs_stack().EndAndPop(
-      Parse::NodeKind::StructLiteralOrStructTypeLiteralStart);
+      Parse::NodeKind::StructLiteralStart);
 
   context.scope_stack().Pop();
   context.node_stack()
-      .PopAndDiscardSoloNodeId<
-          Parse::NodeKind::StructLiteralOrStructTypeLiteralStart>();
+      .PopAndDiscardSoloNodeId<Parse::NodeKind::StructLiteralStart>();
   auto type_block_id = context.args_type_info_stack().Pop();
   if (DiagnoseDuplicateNames(context, type_block_id, "struct literal")) {
     context.node_stack().Push(node_id, SemIR::InstId::BuiltinError);
@@ -115,14 +118,11 @@ auto HandleStructLiteral(Context& context, Parse::StructLiteralId node_id)
 auto HandleStructTypeLiteral(Context& context,
                              Parse::StructTypeLiteralId node_id) -> bool {
   auto refs_id = context.param_and_arg_refs_stack().EndAndPop(
-      Parse::NodeKind::StructLiteralOrStructTypeLiteralStart);
+      Parse::NodeKind::StructTypeLiteralStart);
 
   context.scope_stack().Pop();
   context.node_stack()
-      .PopAndDiscardSoloNodeId<
-          Parse::NodeKind::StructLiteralOrStructTypeLiteralStart>();
-  // This is only used for value literals.
-  context.args_type_info_stack().Pop();
+      .PopAndDiscardSoloNodeId<Parse::NodeKind::StructTypeLiteralStart>();
 
   CARBON_CHECK(refs_id != SemIR::InstBlockId::Empty)
       << "{} is handled by StructLiteral.";