Stop special-casing empty structs. (#2849)

This removes special-casing of empty structs, handling them as just a regular value instead of a builtin. Note the `{} as Type` is still special-cased.

In lowering, removes the test of calling a function using `{}` because it's missing the proper load/store. This setup notices that error whereas the prior worked due to said special-casing. Fixing this will need to be done as part of generally adding loads for variable uses.

toolchain/semantics/semantics_ir.cpp

@@ -166,7 +166,10 @@ auto SemanticsIR::StringifyNode(SemanticsNodeId node_id) -> std::string {
     switch (node.kind()) {
       case SemanticsNodeKind::StructType: {
         auto refs = GetNodeBlock(node.GetAsStructType());
-        if (step.index == 0) {
+        if (refs.empty()) {
+          out << "{} as Type";
+          break;
+        } else if (step.index == 0) {
           out << "{";
         } else if (step.index < static_cast<int>(refs.size())) {
           out << ", ";