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

[Jan Patrick Lehr via llvm-commits]

jplehr wrote:

I tested this patch with the CMake cache file in `offload/cmake/caches/AMDGPUBot.cmake` and see errors about some nvptx that is built. The cache only enables `host;AMDGPU`. Does this patch miss / remove guard?

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