c++: beware of Visual C++ limitations

Contrary to what commit d8cc6b073e "c++:
shorten the assertions that check whether tokens are correct"
believed, MS Visual C++'s preprocessor limitation is not on the input
line length, but on the size of the line holding the full C++
statement.

Reported by Vince Huffaker <vince@vincejulie.com>
<https://lists.gnu.org/r/help-bison/2021-08/msg00003.html>

* data/skeletons/variant.hh (_b4_symbol_constructor_define): Hide the
assertion from Visual C++.

data/skeletons/variant.hh

@@ -484,7 +484,9 @@ m4_define([_b4_symbol_constructor_define],
                               b4_locations_if([l]))[)
 #endif
       {]b4_parse_assert_if([[
+#if !defined _MSC_VER || defined __clang__
         ]b4_assert[ (]b4_tok_in($@)[);
+#endif
       ]])[}
 ]])])