Fix up the type to build with Swift 4.2 also.
Fix up the type to build with Swift 4.2 also.

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