Fixed correctness and crashing bugs in FileManager overlay
1. Use correct URLs in thunk
2. Pass NSString? to thunk, not String?
3. Use proper Objective-C out parameter coding practices
4. Use correct nullability decorator in thunk for backupItemName parameter
rdar://problem/28755011
(cherry picked from commit 66ae38c5e012e8fc82d5c367c08b473ee20c10a0)
