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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> Out of curiosity, why does the existing mechanism not suffice?

it "works" but creates the tag at the beginning of the text rather than the beginning of the attribute documentation. The result is that the scroll target for linking is the beginning of the text rather than the documentation as well, e.g. the location pointed to here:
<img width="1070" alt="Tag+Scroll Location" src="https://github.com/user-attachments/assets/9a003cd6-3f29-4d4c-85b1-8b37d64467bf">

As a result when you follow the link the actual attribute name, syntax table, etc is not visible and it is necessary to scroll to see the actual documentation being linked. 

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