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

[Fangrui Song via cfe-commits]

MaskRay wrote:

(I am not familiar with macOS and I'll defer the decision to Apple folks.
I agree with James that this PR might break users.

You probably want a different way to handle default sysroot in homebrew.
)

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