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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

I guess I'll see how much I favor this approach depending on how much more difficult it is to build the DeviceRTL without OpenMP. I think the only thing we'd miss is the `#pragma omp assumes(...)` business, which might have another way to be emitted in LLVM/Clang?

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