[clang] [Darwin][Driver][clang] Prioritise command line args over `DEFAULT_SYSROOT` (PR #115993)

[Carlo Cabrera via cfe-commits]

carlocab wrote:

> The PR description (first comment), which will be used as the default commit message, should be fixed.

Thanks for the reminder, fixed.

I plan to do a few more local tests with `DEFAULT_SYSROOT` enabled before merging (since this isn't tested in CI).

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