[clang] [openmp] [OpenMP] Respect LLVM per-target install directories (PR #83282)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> @jdoerfert I would like to see the device code compilation (on device runtime) and host runtime compilation fully separate. Then I can build the runtime with gcc or sanitizer without disturbing device code compilation.

Could you elaborate on this? One of my long-term goals is to build the OpenMP device runtime similarly to how I build the GPU C library. That is, we have a `runtimes` build that simply does `--target=amdgcn-amd-amdhsa` on C/C++ source using standard `clang`. We could maybe split this out into a separate runtime and declare the offloading runtime on the CPU to be a project instead?

https://github.com/llvm/llvm-project/pull/83282