[clang] [HIP] fix HIP detection for /usr (PR #80190)

[Yaxun Liu via cfe-commits]

yxsamliu wrote:

> Do we have any tests for this kind of stuff? We really should have some mock ROCm installation in one of the `Inputs/` directories and then do `--rocm-path=` or something.

test added

https://github.com/llvm/llvm-project/pull/80190