[clang] [llvm] [OpenMP] Use generic IR for the OpenMP DeviceRTL (PR #119091)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> I like this method, but just out of curiosity, did we use anything in AMDGPU implementation that has target dependent lowering in the front end? If not, this is totally fine I'd say.

We used to use the `__AMDGCN_WAVEFRONT_SIZE` but that was removed for unrelated reasons.

https://github.com/llvm/llvm-project/pull/119091