[clang] [Headers] Create stub spirv64intrin.h (PR #131164)

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

> Name should be `spirvintrin.h` but if we can't support 32 for now just error in the preprocessor.

Yep, you're right. It'll be caught by only checking for the SPIRV64 macro, but nothing in this file is 32 vs 64 bit dependent as that's part of what gpuintrin.h gives us. Renamed.

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