[clang] [Darwin][Driver][clang] Prioritise `-isysroot` over `--sysroot` consistently (PR #115993)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

Independently of the rationale for the current behaviour, I would be extremely concerned about potential breakage from this behavior change (more due to hyrum's law that intentional choices)

https://github.com/llvm/llvm-project/pull/115993