Apply modernize-use-override (2nd iteration)

Summary:
Use C++11’s override and remove virtual where applicable.
Change are automatically generated.

Reviewed By: shixiao

Differential Revision: D14089910

fbshipit-source-id: 12c776ea06094ee77c7e07263397dab3419c5309

folly/executors/test/TimekeeperScheduledExecutorTest.cpp

@@ -46,7 +46,7 @@ class ManualTimekeeper : public folly::Timekeeper {
   explicit ManualTimekeeper(Executor::KeepAlive<Executor>&& executor)
       : executor_(std::move(executor)), now_(steady_clock::now()) {}
 
-  virtual Future<Unit> after(Duration dur) override {
+  Future<Unit> after(Duration dur) override {
     auto contract = folly::makePromiseContract<Unit>(executor_.get());
     if (dur.count() == 0) {
       contract.first.setValue(folly::unit);