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

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

Well it's not pretty, but spirv64-amd-amdhsa sets both __AMDGPU__ and __SPIRV64__ macros. Added a commit with an example that dispatches to amdgpu intrinsics on the happy path and preprocessor error otherwise.

If you let that get to the spirv backend it falls over with `Intrinsic selection not implemented` but you can have IR with the right amdgpu intrinsics in it.


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