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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> Also, if/when this is merged, it would be nice if someone could go through the rest of the docs and apply this wherever possible (assuming there are any other places where we still need to do that).

yeah, I wanted to isolate this for now to just the attribute docs as these are the only cases I could find, and I found them while working on the docs for #117428.

I'm kind of surprised I couldn't find other labels anywhere, but it could just be because other docs don't reference each other due to the inability to easily do so?

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