[clang] [RFC][Docs][Clang][AMDGPU] Add AMDGPU builtins documentation (PR #181193)

[Shilei Tian via cfe-commits]

shiltian wrote:

> I wonder if we could put doc strings in the builtins.td file itself.

That'd be convenient, but I'm not sure how to generate a documentation like this out of it.

> I'm not sure if we need this for every single one.

We don't have to explain what each builtin does, but I think it definitely makes sense to tell what each argument is for. That is where AI is very convenient, especially we did a fairly good job in the intrinsic definition where we put comments on what each argument is for what, such that AI itself doesn't have to guess or hallucinate. 

https://github.com/llvm/llvm-project/pull/181193