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

[Carlo Cabrera via cfe-commits]

carlocab wrote:

> 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.

I think the issue is that the implicit `-isysroot` flag from `libxcselect` changes the deployment target, which then also changes the behaviour of `clang` in these tests. But I'll need to verify this locally to make sure.

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