c++: provide a means to clear symbols

The symbol destructor is currently the only means to clear a symbol.
Unfortunately during error recovery we might have to clear the
lookahead, which is a local variable (yyla) that has not yet reached
its end of scope.

Rather that duplicating the code to destroy a symbol, or rather than
destroying and recreating yyla, let's provide a means to clear a
symbol.

Reported by Antonio Silva Correia, with an analysis from Michel d'Hooge.
<http://savannah.gnu.org/support/?108481>

* data/c++.m4, data/lalr1.cc (basis_symbol::clear, by_state::clear)
(by_type::clear): New.
(basic_symbol::~basic_symbol): Use clear.

data/lalr1.cc

@@ -288,6 +288,9 @@ b4_location_define])])[
       /// Copy constructor.
       by_state (const by_state& other);
 
+      /// Record that this symbol is empty.
+      void clear ();
+
       /// Steal the symbol type from \a that.
       void move (by_state& that);
 
@@ -542,12 +545,19 @@ m4_if(b4_prefix, [yy], [],
     : state (other.state)
   {}
 
+  inline
+  void
+  ]b4_parser_class_name[::by_state::clear ()
+  {
+    state = empty_state;
+  }
+
   inline
   void
   ]b4_parser_class_name[::by_state::move (by_state& that)
   {
     state = that.state;
-    that.state = empty_state;
+    that.clear ();
   }
 
   inline