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

[Steven Wu via cfe-commits]

cachemeifyoucan wrote:

> Is there any reason why default builds don't just use the macOS spelling, at least for modern versions of macOS?

I don't think anyone has looked into changing the host guess script. I am ok to switch to macOS spelling.

> 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