[clang] clang: Do not pass an empty -syslibroot for --sysroot= on Darwin (PR #229421)

[Steven Wu via cfe-commits]

cachemeifyoucan wrote:

> Prefer configuration files to DEFAULT_SYSROOT.

Are you suggesting we should update tests to use DEFAULT_SYSROOT when possible? That is an option too, as long as we also convert at least one PR testing build job to use a non-standard DEFAULT_SYSROOT so it catches mistakes in driver tests. 

If so, I am not against this patch in its current state.

https://github.com/llvm/llvm-project/pull/229421