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

[Shilei Tian via cfe-commits]

shiltian 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 while the code is not wrapped into `target` region. 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.

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