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

[Michael Buch via cfe-commits]

Michael137 wrote:

> > Any chance we can run the LLDB test-suite on macOS against this change? Happy to help if you haven't got that set up
> 
> Is there a way to do that in CI? I don't have any lldb stuff set up locally, would you or another lldb engineer mind running that if there's no CI way to do it?

Yea no worries. Firing up a build right now. Will update once done

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