message: [c++] Fix To::UnknownField to do not use ellipses
diff: @@ -454,7 +454,8 @@ public:
  void UnknownEnd() const
  {}
  - bool UnknownField(...) const
  + template <typename X>
  + bool UnknownField(uint16_t /*id*/, const X& /*value*/) const
  {
  return false;
  }
  