Fix CaseIterable support for deeply nested enums.
Fix CaseIterable support for deeply nested enums.

Fixes #922.

cburrows/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,cburrows/swift-protobuf,tbkka/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,tbkka/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,cburrows/swift-protobuf,tbkka/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,cburrows/swift-protobuf,tbkka/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf