[nullsafe][annotation graph] Issue a provisional violation in assignments between two provisionally nullables
[nullsafe][annotation graph] Issue a provisional violation in assignments between two provisionally nullables

Summary:
This change is what makes the annotation graph a _graph_.
Now we can detect places when adding one annotation causes an issue that can be
fixed by adding another annotation.

Reviewed By: artempyanykh

Differential Revision: D24650979

fbshipit-source-id: b8452b822

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