If your going this way you must also take account for "reimplementation from scratch under restrictive licence”.
Apache license don’t say anything about clean room reimplementation under other licence because it’s a tricky subject beyond the scope of the license. I personally think that if someone reimplement Swift, name it Schwift, guarantee it’s 200% faster (for some reasons), and then sell commercial licence of that it should be sued (but with such business plan I believe he’ll go bankrupt first).
Apple logically won’t comment on what they will do in case of a "clean room" implementation because they have zero interest in anybody doing such a thing.
And actually nobody have any interest in such a monstrosity (except maybe for theological reasons?). The project is goddam Appache2, fork it if you want, but spending any second trying to reimplement years of works is a total waste of time. Want to start something from scratch? Fine but do something original. Want to improve swift ecosystem ? Fine but work on the main project or on a clean fork!