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

[Steven Wu via cfe-commits]

cachemeifyoucan wrote:

Current patch breaks test:
```
// We pass --sysroot="" to defeat any -DDEFAULT_SYSROOT parameter.
```
I think that just a pure hack in the test case, we can just update the test. @cyndyishida what do you think about the current approach?

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