[clang] [OpenMP] Implicitly include the 'cgpu' and 'mgpu' libraries for OpenMP (PR #67557)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> If I don't build GPU libc, will it cause a link error?

No, that's the purpose of the check for the `llvm-libc-decls` in the compiler resource directory. The presence of that sets the default value to be true, otherwise it's false. You can override the default value with the `-gpulibc` and `-nogpulibc` options that I added.

https://github.com/llvm/llvm-project/pull/67557