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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> Fixed the issue [0fa04b6](https://github.com/llvm/llvm-project/commit/0fa04b6e2cd2169a8e3d22ae879394dbf07c0466) Unrelated to building as projects.

Hah, I probably should've noticed that. Explains why I didn't notice because I always have tests enabled.

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