clang:dev rejects vector<atomic<T>> in EventBaseTest.cpp.

clang:dev rejects vector<atomic<T>> in EventBaseTest.cpp.

Summary: [Folly] clang:dev rejects vector<atomic<T>> in EventBaseTest.cpp.

Test Plan: Build and run `folly/io/async/test/EventBaseTest.cpp` using the current `clang`; try building with `clang:dev` and verifying that at least the related failures are gone.

Reviewed By: meyering@fb.com

Subscribers: mathieubaudet, folly-diffs@, yfeldblum, dougw, brettp

FB internal diff: D1848749

Tasks: 6244720

Signature: t1:1848749:1423866420:7b7354d4568b6a6d6a824236ae8d271f6855d90b