Add @_versioned attribute to _MutableHandle._pointer

Found related errors at https://ci.swift.org/view/Pull%20Request/job/swift-corelibs-xctest-PR-osx/154/consoleFull#-824480375ee1a197b-acac-4b17-83cf-a53b95139a76.

Author: ikesyo

Foundation/Boxing.swift

@@ -14,7 +14,7 @@
 ///
 /// Note: This assumes that the result of calling copy() is mutable. The documentation says that classes which do not have a mutable/immutable distinction should just adopt NSCopying instead of NSMutableCopying.
 internal final class _MutableHandle<MutableType : NSObject> where MutableType : NSCopying {
-    fileprivate var _pointer : MutableType
+    @_versioned fileprivate var _pointer : MutableType
 
     init(reference : MutableType) {
         _pointer = reference.copy() as! MutableType