swift234

SILGen/IRGen: Don't emit redundant witness thunks for @objc protocol witnesses that are already @objc.

If an extension introduces an @objc protocol conformance using existing methods that are already @objc, then we shouldn't emit @objc thunks for them again, and indeed, we can't for native ObjC classes. Fixes <rdar://problem/16330856>.

Swift SVN r15087