This crashed until recently, and although I couldn't find a case where
the RHS had a null type anymore, I also couldn't see a reason it
couldn't happen, since we don't always assign types to every expression
in code-completion type-checking.
rdar://problem/23111219
Swift SVN r32793
We use an enum to describe the relationship between a code completion result
and the expected type at the code completion site. We can use this relationship
to prioritize candidates on the SourceKit side.
Swift SVN r31371
When users complete the right-hand side of an assignment expression, we only
show the results whose types are convertible to those of the left-hand side.
Swift SVN r31357
