V3LockGuard: Add constructor for adopting already locked mutex.

[kamilrakoczy]

Pre-PR to: #4228

Add constructor for adopting already locked mutex.

[wsnyder]

I don't see where this is used, I'm ok with the change, but would prefer not to add dead code that hurts internal coverage. Can you fold this into whatever needs it - or better add a self test case that uses it. Thanks

[kamilrakoczy]

I don't see where this is used, I'm ok with the change, but would prefer not to add dead code that hurts internal coverage. Can you fold this into whatever needs it - or better add a self test case that uses it. Thanks

Sure, I've added usage of it, so now this PR requires: #4477

I think I can't really add selfTest for adopt_lock_t as it would require check if mutex is locked, but std::mutex doesn't have such feature. We could try to use try_lock, but it have undefined behavior when mutex is locked by the same thread that is using try_lock: https://en.cppreference.com/w/cpp/thread/mutex/try_lock. We could also add extra field to V3Mutex class that stores thread id of thread (if mutex is locked) but I think it is not worth just for the test.

[kamilrakoczy]

This PR should be ready for review and merge.