%merge: fix compatibility with api.value.type=union

Reported by Jot Dot.
https://lists.gnu.org/r/help-bison/2020-12/msg00014.html

* data/skeletons/glr.c, data/skeletons/glr2.cc (b4_call_merger): Use
the symbol's slot, not its type.
* examples/c/glr/c++-types.y: Use explicit per-symbol typing together
with api.value.type=union.
(yylex): Use yytoken_kind_t.

data/skeletons/glr.c

@@ -150,7 +150,7 @@ m4_define([b4_rhs_location],
 m4_define([b4_call_merger],
 [b4_case([$1],
          [    b4_symbol_if([$3], [has_type],
-                           [yy0->b4_symbol($3, type) = $2 (*yy0, *yy1);],
+                           [yy0->b4_symbol($3, slot) = $2 (*yy0, *yy1);],
                            [*yy0 = $2 (*yy0, *yy1);])])])