[clang] [Headers] Implement spirvamdgcnintrin.h (PR #131164)

[Nick Sarnie via cfe-commits]

sarnex wrote:

> @sarnex I'm assuming you're somewhat familiar with SPIR-V and the backend. We really need a way to call SPIR-V builtins directly, i.e. some way to get clang to lower something like __builtin_spirv_ballot() to OpGroupNonUniformBallot. I don't know how much SPIR-V exposes for actual 'subgroup' level operations.

I haven't worked on the backend itself, @michalpaszkowski @VyacheslavLevytskyy any ideas on what we can do?

https://github.com/llvm/llvm-project/pull/131164