Non-exhaustive enums: Cut down SE-0192 per core team feedback
Non-exhaustive enums: Cut down SE-0192 per core team feedback

The biggest difference is that it will no longer apply to Swift enums
defined outside the stdlib and overlays; as such, it no longer
provides a spelling for declaring frozen enums. The feature previously
known as `unknown case` is now just spelled `unknown` and can match
any pattern containing a non-frozen enum, or even a pattern containing
an "implicitly frozen enum" from a third-party library (to help with
gradual adoption of changes within a project). See the list of changes
from the first proposal for more information.

anandabits/swift-evolution,ddunbar/swift-evolution,allevato/swift-evolution,jckarter/swift-evolution,natecook1000/swift-evolution,abertelrud/swift-evolution,Nirma/swift-evolution,Nirma/swift-evolution,airspeedswift/swift-evolution,abertelrud/swift-evolution,allevato/swift-evolution,anandabits/swift-evolution,apple/swift-evolution,jckarter/swift-evolution,apple/swift-evolution,rballard/swift-evolution,natecook1000/swift-evolution,hartbit/swift-evolution,DevAndArtist/swift-evolution,DevAndArtist/swift-evolution,hartbit/swift-evolution,airspeedswift/swift-evolution,ddunbar/swift-evolution,rballard/swift-evolution