CFError: fix compile warning

`CFError` conforms to `Error` and contains a `public var _userInfo`.

In
swiftlang/swift@77a3609
the type of this var was changed from `Any?` to `AnyObject?` but this
change was not made in SCLF too.

Consequently there is currently a compiler warning:

> Var '_userInfo' nearly matches defaulted requirement '_userInfo' of
> protocol 'Error'

Fix this warning by adjusting the type of the `_userInfo` var.

Author: ianpartridge

Foundation/NSError.swift

@@ -395,8 +395,8 @@ extension CFError : Error {
         return CFErrorGetCode(self)
     }
 
-    public var _userInfo: Any? {
-        return CFErrorCopyUserInfo(self) as Any
+    public var _userInfo: AnyObject? {
+        return CFErrorCopyUserInfo(self) as AnyObject
     }
 }