Rebase master; move this to use NSSet, not AnyObject.

Author: millenomi

stdlib/public/Darwin/Foundation/NSSet.swift

@@ -17,7 +17,7 @@ extension Set {
   ///
   /// The provided `NSSet` will be copied to ensure that the copy can
   /// not be mutated by other code.
-  fileprivate init(_cocoaSet: __shared AnyObject) {
+  fileprivate init(_cocoaSet: __shared NSSet) {
     assert(_isBridgedVerbatimToObjectiveC(Element.self),
       "Set can be backed by NSSet _variantStorage only when the member type can be bridged verbatim to Objective-C")
     // FIXME: We would like to call CFSetCreateCopy() to avoid doing an