fix generalizer unsoundness
I ended up getting confused while trying to flip the variances when flipping the order. Should be all right now.
This is only exploitable when generalizing if the `ambient_variance` of the relation is `Contravariant`. This can currently only be the case in the NLL generalizer which only rarely generalizes, causing us to miss this regression. Very much an issue with #121462 however.
7ae95b266e