swift234

Properly account for nullability of Objective-C methods with a related result type.

We were losing the nullability of factory methods that turned into
initializers (which should have been mapped to failability), and more
generally, methods with nullability for related result types
(instancetype in Clang, dynamic Self in Swift).

Note that this commit also contains tests for a similar Clang-side fix
where Clang itself wasn't correctly recording that an Objective-C
method with a nullability-qualified instancetype result type in fact
had a related result type.

Found by browsing through the most excellent Swift SDK Analyzer.

Swift SVN r23834