Revert "Remove old auto_serialize2 code (needs snapshot)"

This reverts commit 0bd6da8a8c.

src/libsyntax/ast_util.rs

@@ -408,6 +408,8 @@ fn dtor_dec() -> fn_decl {
 
 #[auto_serialize]
 #[auto_deserialize]
+#[auto_serialize2]
+#[auto_deserialize2]
 type id_range = {min: node_id, max: node_id};
 
 fn empty(range: id_range) -> bool {