[clang] [llvm] [clang][driver][darwin] Base the system prefix on the SDK, not the target (PR #171970)

[Cyndy Ishida via cfe-commits]

https://github.com/cyndyishida commented:

If system prefixes are now derived from the sdk, what happens if one builds against the sdk but doesn't set `isysroot`. I know thats not recommended, but if there exists any clients it would be good account for this if updating clients doesn't scale.

https://github.com/llvm/llvm-project/pull/171970