rustc_span: Ident::invalid -> Ident::empty

The equivalent for `Symbol`s was renamed some time ago (`kw::Invalid` -> `kw::Empty`), and it makes sense to do the same thing for `Ident`s.

compiler/rustc_builtin_macros/src/derive.rs

@@ -85,7 +85,7 @@ impl MultiItemModifier for Expander {
 fn dummy_annotatable() -> Annotatable {
     Annotatable::GenericParam(ast::GenericParam {
         id: ast::DUMMY_NODE_ID,
-        ident: Ident::invalid(),
+        ident: Ident::empty(),
         attrs: Default::default(),
         bounds: Default::default(),
         is_placeholder: false,