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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Okay I don't think there's a solution for builtins but there is one for attributes. Honestly I think it would be nice to have a default documentation string for the builtins but that's probably a little out of scope.

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