Doing so will release all BDD variables used by automata created for
syntactic implication.  This way the main translation will create
acceptance variables again in a more natural order, which will help
the degeneralization (until we get a better degeneralization).

* src/ltlvisit/contain.cc, src/ltlvisit/contain.hh
(language_containment_checker::clear): New method to clear the
containment cache.
* src/ltlvisit/simplify.cc, src/ltlvisit/simplify.hh
(clear_as_bdd_cache): Also call language_containment_checker::clear.