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/contain.hh

@@ -54,6 +54,9 @@ namespace spot
 
       ~language_containment_checker();
 
+      /// Clear the cache.
+      void clear();
+
       /// Check whether L(l) is a subset of L(g).
       bool contained(const formula* l, const formula* g);
       /// Check whether L(!l) is a subset of L(g).