c++: don't define variant<S>, directly define semantic_type

Instead of defining yy::variant<S> and then alias
yy::parser::semantic_type to variant<sizeof (union_type)>, directly
define yy::parser::semantic_type.

This model is more appropriate if we want to sit the storage on top of
unions in C++11.

* data/variant.hh (b4_variant_define): Specialize and inline the
definition into...
(b4_value_type_declare): Here.
Define union_type here.
* data/lalr1.cc: Adjust.

data/lalr1.cc

@@ -178,8 +178,6 @@ m4_define([b4_shared_declarations],
 ]b4_bison_locations_if([m4_ifndef([b4_location_file],
                                   [b4_location_define])])[
 
-]b4_variant_if([b4_variant_define])[
-
   /// A Bison parser.
   class ]b4_parser_class_name[
   {