Better support method reference parameters in generic method inference

[agrieve]

A variant of #1290:

@NullMarked
class Foo {
  interface Callback<T extends @Nullable Object> {
    void onResult(T thing);
  }

  void receiver(Callback<@Nullable Object> value) {
  }

  void target(@Nullable Object thing) {
  }

  public <T extends @Nullable Object> Callback<T> makeCancelable(Callback<T> callback) {
    return callback;
  }

  void main() {
    receiver(makeCancelable(this::target));
  }
}

results in:

Foo.java:21: warning: [NullAway] incompatible types: Callback<Object> cannot be converted to Callback<@Nullable Object>
    receiver(makeCancelable(this::target));
                           ^
    (see http://t.uber.com/nullaway )

An explicit cast will work-around it:

  void main() {
    receiver(makeCancelable((Callback<@Nullable Object>) this::target));
  }