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

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

We don't need a way to call the builtins. See for example this pull request. Intel have done the spirv64-intel- thing similar to the spirv64-amd-amdhsa so they can (presumably?) use whatever intrinsics they like, using another header quite like this one.

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