[clang] [Headers] Create stub spirvintrin.h (PR #131164)

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

OK, we can't have spirv64-unknown-unknown at this point. But we could get this compiling using a mixture of spirv intrinsics (where they exist) and amdgpu intrinsics for spirv64-amd-amdhsa, leaving a preprocessor error for other targets. I'll take a stab at that.

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