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

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

The utility is having a place to fill things in incrementally as we get them working, and thus use libc to drive the implementation of enough of spirv to get things working. If you decline to have any spirv code until everything is working I'll have to do the testing somewhere else.

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