[concurrency][c++] don't generate errors on guards without mutex
[concurrency][c++] don't generate errors on guards without mutex

Summary: `std::unique_lock<...> bla()` is a valid call but which associates the guard object with no mutex. Silence the errors on such calls.

Reviewed By: da319

Differential Revision: D29937496

fbshipit-source-id: 7600200db

jvillard/infer,facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer