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

[Carlo Cabrera via cfe-commits]

carlocab wrote:

> We also introduce an environment variable `CLANG_NO_XCSELECT` that disables this behaviour if Clang is configured with `CLANG_USE_XCSELECT=ON`. This is needed to avoid breaking tests.

It might also be nicer to set this only for the tests that break instead of globally.

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