[clang] [SPIR-V] Initial support for SPIR-V in `gpuintrin.h` (PR #174910)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

This will fail tests until the dependent PRs are merged. Inspecting the basic IR makes sense but I have no way to test this. Hopefully @sarnex can help here in the future because this should make porting the OpenMP support much easier.

The SPIR-V intrinsics are missing thread syncs, an exit, and the pointer introspections. No clue if I got the address spaces or the thread -> grid accessors right.

https://github.com/llvm/llvm-project/pull/174910