Fix a bug in mutex_try_to_lock. This was previously trying to unlock a mutex...

Fix a bug in mutex_try_to_lock.  This was previously trying to unlock a mutex that it didn't own, causing an assertion failure in mutex.cpp.  The issue was that the unique_lock went out of scope, releasing the lock on m, then m.unlock() was called on an already-unlocked mutex.  

This change removes the spurious m.unlock() call.  

If this test was previously passing for anyone with assertions enabled, then they should investigate bugs in their pthread implementation, as pthread_unlock() should not return 0 if the mutex is currently unlocked.

llvm-svn: 175506