[analyzer] Record nullability implications on getting items from NSDictionary

If we get an item from a dictionary, we know that the item is non-null
if and only if the key is non-null.

This patch is a rather hacky way to record this implication, because
some logic needs to be duplicated from the solver.
And yet, it's pretty simple, performant, and works.

Other possible approaches:

 - Record the implication, in future rely on Z3 to pick it up.
 - Generalize the current code and move it to the constraint manager.

rdar://34990742

Differential Revision: https://reviews.llvm.org/D50124

llvm-svn: 339482