[clang] [OpenMP] Allow GPUs to be targeted directly via `-fopenmp`. (PR #122149)

[Krzysztof Parzyszek via cfe-commits]

kparzysz wrote:

To me this looks like compilation for a host, except the GPU is the host.  The only functions that could be called from such a CU would be the top-level ones, not any of the auto-generated one.

Additionally, the host wouldn't support offload, so we'd need to do something about TARGET (or any other construct that cannot be inside of TARGET).  We should probably ignore those with a diagnostic.

https://github.com/llvm/llvm-project/pull/122149