swift234

[ClangImporter] Don't mirror a protocol's methods if they're in the superclass.

Right now we have a hack to mirror ObjC protocol methods into an imported
ObjC class, because Swift's lookup only looks into superclasses, not into
protocols (by default, anyway). This already isn't great because it's wrong
for @optional, but it was particularly bad if /two/ superclasses conformed
to the same protocol (directly or indirectly). Because the mirrored methods
weren't marked as overrides, the type checker would consider them both as
possibilities, leading to strange errors like this:

  <REPL Input>:1:1: error: ambiguous use of 'description'
  w.description()
  ^
  AppKit.NSWindow:268:8: note: found this candidate
    func description() -> String
         ^
  ObjectiveC.NSObject:72:8: note: found this candidate
    func description() -> String
         ^

Now, we check to see if a superclass conforms to a protocol already before
mirroring its methods into the current class or category.

<rdar://problem/16102321> and possibly also <rdar://problem/16038085>

Swift SVN r14189