[sledge] Fix a few existential context fumbles
[sledge] Fix a few existential context fumbles

Summary:
In a few places, implicitly witnessed existential variables could
remain in the existential context. This led to weakness in the solver,
where occurrences bound by the existential context would be not known
to be constrained by their witnesses.

Reviewed By: jvillard

Differential Revision: D20248542

fbshipit-source-id: 44f62839c

facebook/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,jvillard/infer,facebook/infer,jvillard/infer,facebook/infer