Error when bridging `mutable func` for an enum

[piercifani]

When trying to export the following code:

public enum Foo {
  case something
  case anotherthing
  public mutating func applyMarketUpdate(_ update: String) { }
}

It fails with the following output:

/Users/piercifani/Desktop/skip-errors/protocols-errors/travel-posters-model/.build/plugins/outputs/travel-posters-model/TravelPostersModel/destination/skipstone/TravelPostersModel/build/swift/plugins/outputs/swift/TravelPostersModel/destination/skipstone/SkipBridgeGenerated/Experiments_Bridge.swift:77:12: error: 'mutating' is not valid on instance methods in classes
/Users/piercifani/Desktop/skip-errors/protocols-errors/travel-posters-model/.build/plugins/outputs/travel-posters-model/TravelPostersModel/destination/skipstone/TravelPostersModel/build/swift/plugins/outputs/swift/TravelPostersModel/destination/skipstone/SkipBridgeGenerated/Experiments_Bridge.swift:77:12: error: 'mutating' is not valid on instance methods in classes
/Users/piercifani/Desktop/skip-errors/protocols-errors/travel-posters-model/.build/plugins/outputs/travel-posters-model/TravelPostersModel/destination/skipstone/TravelPostersModel/build/swift/plugins/outputs/swift/TravelPostersModel/destination/skipstone/SkipBridgeGenerated/Experiments_Bridge.swift:264:16: error: cannot use mutating member on immutable value: 'peer_swift' is a 'let' constant