Finish refactoring Push/Pop for stronger type handling. (#2987)

This adds a distinction between Unused and SoloParseNode, rather than
equating the two. This is intended to help identify nodes which are
getting pushed but maybe don't need to be.

Not totally done because I want to adjust declaration name handling due
to a quirk with how it mixes Name with Expression, but almost done. Once
that's done the type punning will be completely gone.

toolchain/semantics/semantics_handle_function.cpp

@@ -111,7 +111,7 @@ auto SemanticsHandleReturnType(SemanticsContext& context,
                                ParseTree::Node parse_node) -> bool {
   // Propagate the type expression.
   auto [type_parse_node, type_node_id] =
-      context.node_stack().PopWithParseNode<SemanticsNodeId>();
+      context.node_stack().PopExpressionWithParseNode();
   auto cast_node_id = context.ExpressionAsType(type_parse_node, type_node_id);
   context.node_stack().Push(parse_node, cast_node_id);
   return true;