[Foundation] Extract NSUndoManager extensions into a seperate file

Non functional change; purely organization/gardening

stdlib/public/SDK/Foundation/NSUndoManager.swift

@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+@_exported import Foundation // Clang module
+
+@_silgen_name("NS_Swift_NSUndoManager_registerUndoWithTargetHandler")
+internal func NS_Swift_NSUndoManager_registerUndoWithTargetHandler(
+  _ self_: AnyObject,
+  _ target: AnyObject,
+  _ handler: @escaping @convention(block) (AnyObject) -> Void)
+
+extension UndoManager {
+  @available(*, unavailable, renamed: "registerUndo(withTarget:handler:)")
+  public func registerUndoWithTarget<TargetType : AnyObject>(_ target: TargetType, handler: (TargetType) -> Void) {
+    fatalError("This API has been renamed")
+  }
+
+  @available(OSX 10.11, iOS 9.0, *)
+  public func registerUndo<TargetType : AnyObject>(withTarget target: TargetType, handler: @escaping (TargetType) -> Void) {
+    // The generic blocks use a different ABI, so we need to wrap the provided
+    // handler in something ObjC compatible.
+    let objcCompatibleHandler: (AnyObject) -> Void = { internalTarget in
+      handler(internalTarget as! TargetType)
+    }
+    NS_Swift_NSUndoManager_registerUndoWithTargetHandler(
+      self as AnyObject, target as AnyObject, objcCompatibleHandler)
+  }
+}