c++: don't use YY_ASSERT at all if parse.assert is disabled

In some extreme situations (about 800 tokens), we generate a
single-line assertion long enough for Visual C++ to discard the end of
the line, thus falling into parse ends for the missing `);`.  On a
shorter example:

    YY_ASSERT (tok == token::TOK_YYEOF || tok == token::TOK_YYerror || tok == token::TOK_YYUNDEF || tok == token::TOK_ASSIGN || tok == token::TOK_MINUS || tok == token::TOK_PLUS || tok == token::TOK_STAR || tok == token::TOK_SLASH || tok == token::TOK_LPAREN || tok == token::TOK_RPAREN);

Whether NDEBUG is used or not is irrelevant, the parser dies anyway.

Reported by Jot Dot <jotdot@shaw.ca>.
https://lists.gnu.org/r/bug-bison/2020-11/msg00002.html

We should avoid emitting lines so long.

We probably should also use a range-based assertion (with extraneous
parens to pacify fascist compilers):

    YY_ASSERT ((token::TOK_YYEOF <= tok && tok <= token::TOK_YYUNDEF)
               || (token::TOK_ASSIGN <= tok && ...)

But anyway, we should simply not emit this assertion at all when not
asked for.

* data/skeletons/variant.hh: Do not define, nor use, YY_ASSERT when it
is not enabled.

NEWS

@@ -4,9 +4,17 @@ GNU Bison NEWS
 
 ** Bug fixes
 
+*** Bug fixes in yacc.c
+
   In Yacc mode, all the tokens are defined twice: once as an enum, and then
   as a macro.  YYEMPTY was missing its macro.
 
+*** Bug fixes in lalr1.cc
+
+  The lalr1.cc skeleton used to emit internal assertions (using YY_ASSERT)
+  even when the `parse.assert` %define variable is not enabled.  It no
+  longer does.
+
 ** Changes
 
   The YYBISON macro in generated "regular C parsers" (from the "yacc.c"