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

[Liviu Ionescu via cfe-commits]

ilg-ul wrote:

Yes, that's why I said that your patch must be elaborated and check for the presence of `-isysroot`, since the internal variable may come from other settings.


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