Reland "[clang][Driver] Correct tool search path priority"

This reverts commit f570d581.

The test was failing on MacOS if you set
LLVM_DEFAULT_TARGET_TRIPLE. For example if you set it to
"x86_64-apple-darwin" clang actually uses
"x86_64-apple-darwin<version>".

To fix this get default triple from clang itself during the
test instead of substituting it in via lit.