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

[Ye Luo via cfe-commits]

ye-luo wrote:

> > @jhuber6 could you build openmp as a project instead of runtime?
> 
> Ah, I could try that. Though I believe that Johannes is going to completely deprecate the projects build once moving to llvm/offload.

@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.

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