[clang] [clang][Driver][Darwin] Optionally use xcselect to find macOS SDK (PR #119670)

[Carlo Cabrera via cfe-commits]

carlocab wrote:

> Need to add test. You should add `CLANG_USE_XCSELECT` as a requirement and label all the tests properly (including those will break). Maybe you can also remove the hack for `CLANG_NO_XCSELECT` but I am neutral on that.

All done, thanks.

I don't quite understand why the failing tests are caused to fail when `CLANG_USE_XCSELECT` is enabled. I'll have to put more time into investigating them -- I consider this another blocker for enabling this feature more widely.

https://github.com/llvm/llvm-project/pull/119670