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

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

@ldionne if downstream is catching upstream, I would love to discuss rationale behind ignoring command line option. I asked this question several times and haven't got answer. Original [diff](https://reviews.llvm.org/D89001) that introduced this behavior also didn't explain why new behavior is "intended" and better than old one. Instead of resolving case of two headers it actually prevents finding header that exists and path is specified in command line.

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