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

[Daniel Rodríguez Troitiño via cfe-commits]

drodriguez wrote:

I agree with Dmitry in that it would be preferred to have the system compiler behaviour until the system compiler actually changes.

I have tried again with Xcode 14.3.1 (released almost 1 year ago), which includes `include/c++/v1` side by side with the compiler, and for it, passing `-isysroot` still overrides the side by side path.

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