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

[Mészáros Gergely via cfe-commits]

Maetveis wrote:

> This function was a little weird because the intrinsic specifies the address space I think. Do you know explicitly why we can't cast here? Seems weird that the compiler would fall over on this.

https://github.com/llvm/llvm-project/pull/152528 explores that a bit. `[[clang::opencl_generic]]` doesn't really make much sense, AFAICT it should be equivalent to the default language address space in SYCL. We could accept it as such, but would have to change the API of `asOpenCLLangAS` / `asSYCLLangAS` / `asHLSLLangAS` because they represent the condition of "not a <language> address space attribute" by returning `LangAS::Default`.

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