[swift2objc] Support mutating methods

[liamappelbe]

struct Point {
  var x: Double
  var y: Double

  mutating func moveBy(x deltaX: Double, y deltaY: Double) {
    x += deltaX
    y += deltaY
  }
}

This came up while I was writing a unit test. These might work without any changes on our end, or we might have to declare the wrapper method as mutating. Need a test for this case either way.