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

[Carlo Cabrera via cfe-commits]

carlocab wrote:

> Maybe instead, the logic in the patch should be setting `-syslibroot` for linker in following order:
> 
> * `--sysroot`
> * `-isysroot`
> * `C.getSysRoot()`, which is basically DEFAULT_SYSROOT as other cases is handled in `--sysroot`

Thanks, pushed a change that I think should implement this. Haven't gotten around to testing it yet, I will this afternoon.

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