mbe: Use TokenTree as the fallback for invalid fragment specifiers

`tt` should match more, so use this for both missing and invalid
fragment specifiers.

Also remove one unneeded instance of `String`.

compiler/rustc_expand/src/errors.rs

@@ -444,7 +444,7 @@ pub(crate) struct InvalidFragmentSpecifier {
     #[primary_span]
     pub span: Span,
     pub fragment: Ident,
-    pub help: String,
+    pub help: &'static str,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_expand/src/mbe/quoted.rs

@@ -117,9 +117,9 @@ pub(super) fn parse(
                 sess.dcx().emit_err(errors::InvalidFragmentSpecifier {
                     span,
                     fragment,
-                    help: VALID_FRAGMENT_NAMES_MSG.into(),
+                    help: VALID_FRAGMENT_NAMES_MSG,
                 });
-                NonterminalKind::Ident
+                NonterminalKind::TT
             });
             result.push(TokenTree::MetaVarDecl { span, name: ident, kind });
         } else {