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_handle_struct.cpp

@@ -74,12 +74,6 @@ auto SemanticsHandleStructLiteral(SemanticsContext& context,
       ParseNodeKind::StructLiteralOrStructTypeLiteralStart);
   auto type_block_id = context.args_type_info_stack().Pop();
 
-  // Special-case `{}`.
-  if (refs_id == SemanticsNodeBlockId::Empty) {
-    context.node_stack().Push(parse_node, SemanticsNodeId::BuiltinEmptyStruct);
-    return true;
-  }
-
   // Construct a type for the literal.
   // TODO: This should try to canonicalize the struct form before adding the
   // node.