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

[Erich Keane via cfe-commits]

erichkeane wrote:

I have no idea what is going on here... @AaronBallman probably needs to take a better look.  Else, could the author please give me an ELI5 sorta thing here or show me what the change looks like on the generated stuff? 

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