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

[Joseph Huber via cfe-commits]

jhuber6 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.

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