c++: fix double free when a symbol_type was moved

Currently the following piece of code crashes (with parse.assert),
because we don't record that s was moved-from, and we invoke its dtor.

    {
      auto s = parser::make_INT (42);
      auto s2 = std::move (s);
    }

Reported by Wolfgang Thaller.
http://lists.gnu.org/archive/html/bug-bison/2018-12/msg00077.html

* data/c++.m4 (by_type): Provide a move-ctor.
(basic_symbol): Be sure not to read a moved-from value.
* tests/c++.at (C++ Variant-based Symbols Unit Tests): Check this case.

THANKS

@@ -180,6 +180,7 @@ Wayne Green               wayne@infosavvy.com
 Wei Song                  wsong83@gmail.com
 Wojciech Polak            polak@gnu.org
 Wolfgang S. Kechel        wolfgang.kechel@prs.de
+Wolfgang Thaller          wolfgang.thaller@gmx.net
 Wolfram Wagner            ww@mpi-sb.mpg.de
 Wwp                       subscript@free.fr
 xolodho                   xolodho@gmail.com