It clearly has not been "designated," hence the confusion in this thread and the issue I linked.
I would hope the Swift compiler would not simply rely on the clang driver to set such important target info, and that the importance of C will fade with time.
Not every single option, just the important ones.
To optimize Swift codegen, you should be setting Swift flags, not old clang flags, particularly given that nobody finds clang's arch-targeting flags that great. If you want, you can just have the Swift compiler translate the Swift flag to the clang equivalent, if that works, like I now see -target-cpu has been doing from the beginning.
But we definitely shouldn't leave such important arch-targeting flags to an undocumented hack of passing it to clang.