[clang] AMDGPU: Find device files even when ROCM_PATH is set (PR #177346)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> Or make clang to use the resource dir in case that ROCM_PATH doesn't contain device libs.

I think this should be the opposite. ROCM_PATH should never overrule the resource directory.

I think the search priority logic should be:
1. Explicit `--rocm-device-lib-path`
2. Resource directory contains libraries. This wins, ignore ROCM_PATH. 
3. ROCM_PATH (or ignore it, though I'm not sure that's going to be easy to just do today)


https://github.com/llvm/llvm-project/pull/177346