[clang] Add support for referencable labels for attribute documentation (PR #118428)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> LGTM after fixing one more thing.
> 
> I personally think this is a reasonable addition now, but please wait for Aaron or Erich to approve this too before merging.

Righto, I've committed the change, is `const StringRef` a style violation (I recognize StringRef is a constant type)? (Pondering adding support for style rule enforcement for things like "this is a constant type so doesn't need to be const")

https://github.com/llvm/llvm-project/pull/118428