[clang] [Clang][Headers] Fix <gpuintrin.h> for SYCL (PR #152314)

[Mészáros Gergely via cfe-commits]

Maetveis wrote:

> You should not be able to hit llvm_unreachable by doing anything with code. Clang needs to be fixed to not assert

Reported as https://github.com/llvm/llvm-project/issues/152460, will fix in a separate PR. I still believe the change here is worth pursuing.

https://github.com/llvm/llvm-project/pull/152314