Clear the contaiment cache after -r7.

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.

src/ltlvisit/simplify.cc

@@ -4294,6 +4294,7 @@ namespace spot
     ltl_simplifier::clear_as_bdd_cache()
     {
       cache_->clear_as_bdd_cache();
+      cache_->lcc.clear();
     }
 
   }