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

[Steven Wu via cfe-commits]

cachemeifyoucan wrote:

I think the correct solution for Homebrew is to never use `--sysroot` and switch to `-isysroot` since the former is almost never used on Darwin platforms.

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