JSpecify: infer diamond type-argument nullability from constructor arguments without a target type

[msridhar]

E.g., for this code:

@NullMarked
public class Test {
  static class Box<T extends @Nullable Object> {
    private final T value;
    Box(T value) {
      this.value = value;
    }
    T get() {
      return value;
    }
  }
  void test() {
    // should report no error
    var inferredFromInitializer = new Box<>(null);
  }
}

We should be able to infer that inferredFromInitializer has type Box<@Nullable Object>; right now we do not. Possibly related to #1532 .