C++: don't issue the definition of symbol_type when not used

Currently, in glr.cc, we emit the definitions of basic_symbol and
symbol_type, although there are not used.

* data/c++.m4 (b4_public_types_declare): Extract these definitions from
here, and move them...
(b4_symbol_type_declare): here.
(b4_public_types_declare): Also remove the definition of the symbol
constructors.
* data/lalr1.cc (b4_shared_declarations): Adjust: call
b4_symbol_type_declare and b4_symbol_constructor_declare.

data/lalr1.cc

@@ -176,6 +176,7 @@ b4_location_define])])[
   {
   public:
 ]b4_public_types_declare[
+]b4_symbol_type_declare[
     /// Build a parser object.
     ]b4_parser_class_name[ (]b4_parse_param_decl[);
     virtual ~]b4_parser_class_name[ ();
@@ -206,6 +207,8 @@ b4_location_define])])[
     /// Report a syntax error.
     void error (const syntax_error& err);
 
+]b4_symbol_constructor_declare[
+
   private:
     /// This class is not copyable.
     ]b4_parser_class_name[ (const ]b4_parser_class_name[&);