[clang] [llvm] [AMDGPU] Introduce asyncmark/wait intrinsics (PR #173259)

[Pierre van Houtryve via cfe-commits]

https://github.com/Pierre-vh commented:

I wish this was split into multiple patches because it's quite hard to review all at once. Not sure how other reviewers feel about it.

Can you update the commit description to give a more accurate overview of all the contents of this patch?

https://github.com/llvm/llvm-project/pull/173259