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

[Liviu Ionescu via cfe-commits]

ilg-ul wrote:

If, for any reason, you need the SDK path to be the first in the search list, I suggest you add an extra condition, the explicit presence of the `-isysroot`, since it looks like the `Sysroot` variable may come from other configuration definitions.

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