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

[Erich Keane via cfe-commits]

erichkeane wrote:

Yeah, agreed, thanks for the ELI5.  When it comes to RST/HTML, I'm roughly 5 :)  LGTM as well.

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