c++: using macros around user types breaks when they include comma

We may generate code such as

    basic_symbol (typename Base::kind_type t, YY_RVREF (std::pair<int,int>) v);

which, of course, breaks, because YY_RVREF sees two arguments.  Let's
not play tricks with _VA_ARGS__, I'm unsure about it portability.
Anyway, I plan to change more things in this area.

Reported by Sébastien Villemot.
http://lists.gnu.org/archive/html/bug-bison/2018-11/msg00014.html

* data/variant.hh (b4_basic_symbol_constructor_declare)
(b4_basic_symbol_constructor_define): Don't use macro on user types.
* tests/types.at: Check that we support pairs.

THANKS

@@ -151,6 +151,7 @@ Roland Levillain          [email protected]
 Satya Kiran Popuri        [email protected]
 Sebastian Setzer          [email protected]
 Sebastien Fricker         [email protected]
+Sébastien Villemot        [email protected]
 Sergei Steshenko          [email protected]
 Shura                     [email protected]
 Simon Sobisch             [email protected]