[llvm] [OpenMP] Port the OpenMP device runtime to direct C++ compilation (PR #123673)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> Is the plan to eventually make CMake handle `--target=${target_triple}` (and include dependencies)? like it does with `LLVM_ENABLE_RUNTIMES`?
> 
> I assume header files that are #included in user programs (e.g. `__clang_openmp_device_functions.h`) wil remain unaffected?

Yes, the idea is to remove all of the hacky compilations and just use `add_library` or `add_executable`. That'll make the build more 'normal'.

https://github.com/llvm/llvm-project/pull/123673