[clang] [SPIRV] Properly discover LLVM tools that live next to the compiler (PR #178779)

[Johannes Doerfert via cfe-commits]

https://github.com/jdoerfert approved this pull request.

This makes the SPIRV OpenMP device runtime build succeed even if the PATH llvm-link is old.

https://github.com/llvm/llvm-project/pull/178779