[lit] Fix testing of standalone clang and lld builds

In such cases, the executables are not in the llvm_tools_dir directory, so we need to look in the other search locations. Previously, they were found via the PATH, but this was disabled by default in commit rGa1e65658.

Depends on D103154.

Reviewed By: thopre

Differential Revision: https://reviews.llvm.org/D103156