Add ErrorGuaranteed to ast::LitKind::Err, token::LitKind::Err.

This mostly works well, and eliminates a couple of delayed bugs.

One annoying thing is that we should really also add an
`ErrorGuaranteed` to `proc_macro::bridge::LitKind::Err`. But that's
difficult because `proc_macro` doesn't have access to `ErrorGuaranteed`,
so we have to fake it.

compiler/rustc_expand/src/base.rs

@@ -1266,7 +1266,7 @@ pub fn expr_to_spanned_string<'a>(
                 );
                 Some((err, true))
             }
-            Ok(ast::LitKind::Err) => None,
+            Ok(ast::LitKind::Err(_)) => None,
             Err(err) => {
                 report_lit_error(&cx.sess.parse_sess, err, token_lit, expr.span);
                 None