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

[Björn Pettersson via cfe-commits]

bjope wrote:

Hi @jhuber6, @MaskRay 

We are having some problems with this patch on a server where the file /lib64/libomptarget-nvptx-sm_52.bc exists.
The test case that fails is clang/test/Driver/openmp-offload-gpu.c.

**Problem 1**
I think one problem is related to this check line
`// CHK-ENV-BCLIB: clang{{.*}}-triple{{.*}}nvptx64-nvidia-cuda{{.*}}-mlink-builtin-bitcode{{.*}}subdir{{/|\\\\}}libomptarget-nvptx-sm_52.bc
`
That test is using `env LIBRARY_PATH` but your changes in `tools::addOpenMPDeviceRTL` makes it prioritize the standard library paths before the environment. Not sure if that is how it should be or if env should have higher prio (i.e. added to LibraryPaths before the paths found in HostTC).

**Problem 2**
This check line also started failing:
`// CHK-BCLIB-WARN: no library 'libomptarget-nvptx-sm_52.bc' found in the default clang lib directory or in LIBRARY_PATH; use '--libomptarget-nvptx-bc-path' to specify nvptx bitcode library
`

Now, with your path, I guess it starts picking up the `/lib64/libomptarget-nvptx-sm_52.bc` file from the system. So we no longer get the warning. Is that the intention with your patch? Regardless, I think you need to do something with that test case because I think the "should never exist" part in
```
/// Check that the warning is thrown when the libomptarget bitcode library is not found.
/// Libomptarget requires sm_52 or newer so an sm_52 bitcode library should never exist.
```
no longer holds with your patch.

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