Setting the interface type of a variable, just to reset it to a null type is actually really gross. But quite a few methods further down in the generation of code completion results (such as USR generation) need to get a variable’s type and passing them along in a separate map would be really invasive. So this seems like the least bad solution to me.
This eliminates a source of bugs if subclasses of `TypeCheckCompletionCallback` forget to call the superclass’s implementation of `sawSolution` from their overridden method.
