[clang] [Darwin][Driver][clang] Prioritise command line args over `DEFAULT_SYSROOT` (PR #115993)

[Cyndy Ishida via cfe-commits]

cyndyishida wrote:

> Sorry, which env var are you referring to?

I'm suggesting an explicit test case that checks that `DEFAULT_SYSROOT` is set and behaves as expected. It's ignored if  sysroot or isysroot is passed, it's used when they're missing. 


https://github.com/llvm/llvm-project/pull/115993