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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> I think that is a misuse of OpenMP semantics. We can't expect to have regular OpenMP code working in the same way as OpenMP offloading code when targeting a GPU meanwhile the code is not wrapped into `target` region or declare target. I understand to have variants and declare target is not convenient but that conforms with OpenMP standard. I don't think this change conforms with the standard.

It should maintain the normal semantics you'd get with `-fopenmp` except it codegens certain things differently. Alternatively I could just remove OpenMP entirely from the DeviceRTL so I might just do that instead.

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