Ensure ann tags are actually kept around during typechecking

This way, the tag assigned by the parser stays with the node.
I realize ann replacing is probably going away real soon, but
I needed this now for moving the resolve defs out of the AST.

src/comp/front/ast.rs

@@ -24,11 +24,19 @@ type ty_param = ident;
 // Annotations added during successive passes.
 tag ann {
     ann_none(uint);
-    ann_type(middle.ty.t,
+    ann_type(uint,
+             middle.ty.t,
              Option.t[vec[middle.ty.t]], /* ty param substs */
              Option.t[@ts_ann]); /* pre- and postcondition for typestate */
 }
 
+fn ann_tag(&ann a) -> uint {
+    ret alt (a) {
+        case (ann_none(?t)) { t }
+        case (ann_type(?t, _, _, _)) { t }
+    };
+}
+
 tag def {
     def_fn(def_id);
     def_obj(def_id);