Don't expose the pthread_mutex_t underlying the Mutex & Mutex::Locker classes.

No one was using it and Locker(pthread_mutex_t *) immediately asserts for 
pthread_mutex_t's that don't come from a Mutex anyway.  Rather than try to make
that work, we should maintain the Mutex abstraction and not pass around the
platform implementation...

Make Mutex::Locker::Lock take a Mutex & or a Mutex *, and remove the constructor
taking a pthread_mutex_t *.  You no longer need to call Mutex::GetMutex to pass
your mutex to a Locker (you can't in fact, since I made it private.)

llvm-svn: 156221