Allow extern mods to be anonymous

extern mod {
  f();
}

is now allowed, and puts f in the enclosing scope. (Requires a
link_name attribute to be really useful...)

src/libsyntax/fold.rs

@@ -554,7 +554,8 @@ fn noop_fold_mod(m: _mod, fld: ast_fold) -> _mod {
 }
 
 fn noop_fold_foreign_mod(nm: foreign_mod, fld: ast_fold) -> foreign_mod {
-    return {view_items: vec::map(nm.view_items, |x| fld.fold_view_item(x)),
+    return {sort: nm.sort,
+         view_items: vec::map(nm.view_items, |x| fld.fold_view_item(x)),
          items: vec::map(nm.items, |x| fld.fold_foreign_item(x))}
 }