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

[Daniel Rodríguez Troitiño via cfe-commits]

drodriguez wrote:

@ilg-ul: you might be interested in reviewing this changes, since they change the behaviour you introduced in #70817 when `-isysroot` is provided.

If some folks at Apple reads this: it would be preferable to have the actual code instead of trying to guess which is the internal behaviour of Apple Clang. I will be happy to close this PR if the original code (or a "censored" version of it) can be upstreamed. Thanks!

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