swift234

[ClangImporter] Don't mirror root class methods returning instancetype.

In Objective-C, all instance methods on a root class are also available as
class methods (because all class objects in Objective-C are instances of the
root class). However, we were incorrectly introducing class methods that
returned 'Self' (instead of 'Self.Type') for every instance method with a
related result type. Returning 'Self.Type' exposes a new type checker bug
<rdar://problem/16414206>, but in practice we don't have much reason to do
this anyway. For now, just don't try to mirror instancetype-returning methods
as class methods.

Swift SVN r15435