`NYI` comment for open generics in `AbstractType.constraintAssignableFrom`

[fridis]

The code contains this:

                result = actual.feature() == feature() &&
                  genericsAssignable(actual, context); // NYI: Check: What about open generics?

Need to check if open type parameter with type constraints may cause any problems. This might be related to #4994 and #4992.