Fix override warnings for clang
Fix override warnings for clang

Summary:
Clang (Apple LLVM version 9.0.0 clang-900.0.37)) complains:
```
"foo" overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
```
Also the linter complains the same issue.

Fixed it. Also did clang-format on this file.

Reviewed By: int3

Differential Revision: D6017138

fbshipit-source-id: e94db0c0705a776b42fdfaba07cbf9bf6f6a7492

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex