Summary:
As title.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9383
Test Plan:
manually add `using namespace` to a file, and run `make check-sources`.
Then, remove `using namespace`, and run `make check-sources`
Reviewed By: ajkr
Differential Revision: D33551706
Pulled By: riversand963
fbshipit-source-id: 1bb8304f38434da7de0656882e62e77673155725