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

[Daniel Rodríguez Troitiño via cfe-commits]

drodriguez wrote:

The `Xcode.app` I was using in the previous comment was 15.1. I tried again the same steps with the last version I have (Xcode 15.3b1, 15E5178i) and I obtain the same behaviour.

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