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

[Carlo Cabrera via cfe-commits]

carlocab wrote:

After excluding the `-darwin*` spelling from using `xcselect`, most of the failures go away, but one remains. I'll investigate it tomorrow.

I've left it as `XFAIL` for now, since it seems to be the least bad option (compared to `CLANG_NO_XCSELECT` or `UNSUPPORTED`), given:

> > I'm wondering if it's worth reviving the CLANG_NO_XCSELECT environment variable and then just setting them in these tests (and slowly getting rid of instances of CLANG_NO_XCSELECT as we find better fixes).
> 
> The downside here is that when investigating running each command, it might be confusing that the error is not reproducing unless an env is set.
> 
> Alternative is just `UNSUPPORTED:` but that will reduce the coverage when XCSELECT is configured.

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