Merge pull request swiftlang#1480 from ikesyo/calendar-_autoupdating-private

[Calendar] `var _autoupdating` could be `private`

Author: ikesyo

Foundation/Calendar.swift

@@ -30,7 +30,7 @@ internal func __NSCalendarInit(_ identifier: String) -> NSCalendar? {
 public struct Calendar : Hashable, Equatable, ReferenceConvertible, _MutableBoxing {
     public typealias ReferenceType = NSCalendar
 
-    internal var _autoupdating: Bool
+    private var _autoupdating: Bool
     internal var _handle: _MutableHandle<NSCalendar>
 
     /// Calendar supports many different kinds of calendars. Each is identified by an identifier here.