[clang] [AMDGPU] Address post-commit review of #177343 (PR #186064)

[Pierre van Houtryve via cfe-commits]

Pierre-vh wrote:

> > OpenCL has no one-as
> 
> Did you mean it has no cluster scope?

Yes sorry, will update the description

> Can we retire the -one-as stuff completely now? I thought it was obsoleted by MMRA

No not really, one-as is something we want to revisit at some point but not yet, and MMRAs don't make strong enough guarantees for it

https://github.com/llvm/llvm-project/pull/186064