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

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

@ldionne what about fixing upstream behavior to match actual system compiler behavior now and when/as soon as we have new behavior in Apple compiler fix it again in newer clang versions? Moreover as I pointed out Apple compiler behavior is problematic for clang-tools and clangd because it basically ignores option passed in command line.

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