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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> Could you explain what each line does exactly?

This is hypothetical, but it's a potential way to keep it from having a separate project
`-DLLVM_RUNTIME_TARGETS=default;amdgcn-amd-amdhsa;nvptx64-nvidia-cuda`
Enables the runtimes for the target triples, default is what you get without specifying anything

-DLLVM_ENABLE_RUNTIMES="openmp" 
This specifies the default target builds the openmp runtime.

-DRUNTIMES_amdgcn-amd-amdhsa_LLVM_ENABLE_RUNTIMES=openmp 
This specifies that the amdgcn-amd-amdhsa target builds the openmp runtime.

Theoretically this would let us treat these as separate builds so you could pass totally different things to them. Just throwing ideas around.

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