stdlib: Make isUniquelyReferenced shims properly return bool.

rdar://problem/18573806 is fixed.

Swift SVN r23547

stdlib/core/Builtin.swift

@@ -241,7 +241,7 @@ internal func _usesNativeSwiftReferenceCounting(theClass: AnyClass) -> Bool {
 #if _runtime(_ObjC)
   return _swift_usesNativeSwiftReferenceCounting_class(
     unsafeAddressOf(theClass)
-  ) != 0
+  )
 #else
   return true
 #endif