Don't acquire the mutex during the destructor of PassRegistry.

This destructor is run as part of static program termination, and
so all ManagedStatics (including this lock) will have been
destroyed by llvm_shutdown.  Furthermore, if there is actually
a race condition during static program termination, then we are
just hiding a bug somewhere else, because other threads should
not be running at this point.

llvm-svn: 210717