c++: minor stylistic changes

* data/c++m4: Remove useless comment lines.
* data/variant.hh (self_type): Use this typedef instead of variant<S>.
(b4_symbol_constructor_define_): Remove commented-out line, and stylistic
change (avoid blank line).

data/c++.m4

@@ -234,7 +234,6 @@ m4_define([b4_public_types_declare],
       enum { empty = 0 };
 
       /// The symbol type.
-      ///
       /// -1 when this symbol is empty.
       token_number_type type;
     };