...instead of relying on the one in the overlay in pre-4.2 versions of
Swift. This caused crashes in deserialization, which (deliberately)
doesn't respect availability.
There are three changes here:
- Remove UIEdgeInsets.zero and UIOffset.zero from the UIKit overlay.
- Always use the 4.2 name for UIEdgeInsetsZero and UIOffsetZero from
the underlying UIKit framework. (This is the nested name.)
- Ignore the unavailability messages for those two constants in
pre-4.2 Swift, since we're now relying on them being present.
The latter two, the compiler changes, can go away once UIKit's API
notes no longer specify different pre-4.2 behavior, but meanwhile we
need to keep compatibility with the SDKs released in Xcode 10b1.
https://bugs.swift.org/browse/SR-7879
