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

[Mészáros Gergely via cfe-commits]

Maetveis wrote:

> it seems weird that C/C++ can use it fine but SYCL can't.

Yes, I agree. It's really not clear if the `opencl_attributes` are meant to be usable outside of OpenCL or not, and if they do why did SYCL give them different semantics than C/C++ in the first place.

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