Rollup merge of #124778 - fmease:fix-diag-msg-parse-meta-item, r=nnet…

…hercote

Fix parse error message for meta items

Addresses rust-lang/rust#122796 (comment), cc [``@]Thomasdezeeuw.``

For attrs inside of a macro like `#[doc(alias = $ident)]` or `#[cfg(feature = $ident)]` where `$ident` is a macro metavariable of fragment kind `ident`, we used to say the following when expanded (with `$ident` ⟼ `ident`):

```
error: expected unsuffixed literal or identifier, found `ident`
  --> weird.rs:6:19
   |
6  |      #[cfg(feature = $ident)]
   |                      ^^^^^^
...
11 | m!(id);
   | ------ in this macro invocation
   |
   = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
```

This was incorrect and caused confusion, justifiably so (see #122796).

In this position, we only accept/expect *unsuffixed literals* which consist of numeric & string literals as well as the boolean literals / the keywords / the reserved identifiers `false` & `true` **but not** arbitrary identifiers.

Furthermore, we used to suggest garbage when encountering unexpected non-identifier tokens:

```
error: expected unsuffixed literal, found `-`
  --> weird.rs:16:17
   |
16 | #[cfg(feature = -1)]
   |                 ^
   |
help: surround the identifier with quotation marks to parse it as a string
   |
16 | #[cfg(feature =" "-1)]
   |                + +
```

Now we no longer do.