Do not optimize "display classes" into struct inside interfaces with variance annotations

[controlflow]

interface I1<T> {
  void M(int parameter) {
    // display class is optimized into struct
    void Local() => parameter++;
    Local();
  }
}

interface I2<out T> {
  void M(int parameter) {
    // display class is a reference type
    void Local() => parameter++;
    Local();
  }
}