Fix a think in Mutex::Locker::Locker(pthread_mutex_t *) Really should lock...

Fix a think in Mutex::Locker::Locker(pthread_mutex_t *)  Really should lock the mutex handed in, not the m_mutex_ptr that you've set to NULL...
Rework the Host.cpp::ThreadNameAccessor to use ThreadSafeSTLMap - we've got it so we might as well use it.  Also works around a problem with the
Mutex::Locker class raising fallacious asserts in debug mode when used with pthread_mutex_t's that weren't backed by Mutex objects.

llvm-svn: 156193