[clang] [clang][SPIRV] Add builtin for OpGenericCastToPtrExplicit and its SPIR-V friendly binding (PR #137805)

[Victor Lomuller via cfe-commits]

Naghasan wrote:

Sorry I can't assign reviewers, @JonChesterfield  @jhuber6 as you are involved in gpuintrin.h, I'd welcome feedbacks here (feel free to ping other relevant persons)

For the SPIR-V side of thing @VyacheslavLevytskyy @farzonl I appreciate your feedbacks as well :)


FYI @tahonermann 

https://github.com/llvm/llvm-project/pull/137805