swift

a83ef229 - [Constraint Solver] Don't perform a join when we've adjusted an IUO to optional.

[Constraint Solver] Don't perform a join when we've adjusted an IUO to optional. Fixes rdar://problem/28621624, an intentional source-breaking change from Swift 3. (cherry picked from commit 4b9adae8ae1ddf6388e64af37cee5376dfda9ecd)