c++: fix the use of destructors when variants are enabled

When using variants, destructors generate invalid code.
<http://lists.gnu.org/archive/html/bug-bison/2014-09/msg00005.html>
Reported by Michael Catanzaro.

* data/c++.m4 (~basic_symbol): b4_symbol_foreach works on yysym:
define it.
* tests/c++.at (Variants): Check it.

data/c++.m4

@@ -346,6 +346,7 @@ m4_define([b4_public_types_define],
   {]b4_variant_if([[
     // User destructor.
     symbol_number_type yytype = this->type_get ();
+    basic_symbol<Base>& yysym = *this;
     switch (yytype)
     {
 ]b4_symbol_foreach([b4_symbol_destructor])dnl