[clang] [clang] Match -isysroot behaviour with system compiler on Darwin (PR #80524)

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

@ldionne I also haven't see Apple compiler that matches upstream behavior. In https://reviews.llvm.org/D157283#4648242, you mentioned that "Xcode 15 RC" should be such compiler and checked it and couple version after that and none of them matched upstream so I decided to wait and forgot about the issue. @drodriguez thank you for rising it again!

https://github.com/llvm/llvm-project/pull/80524