variants: prohibit simple copies

The "variant" structure provides a means to store, in a typeless way,
C++ objects.  Manipulating it without provide the type of the stored
content is doomed to failure.  So provide a means to copy in a type
safe way, and prohibit typeless assignments.

* data/c++.m4 (symbol_type::move): New.
* data/lalr1.cc: Use it.
* data/variant.hh (b4_variant_define): Provide variant::copy.
Let variant::operator= abort.
We cannot undefine it, yet, as it is still uses by the implicit
assigment in symbols, which must also be disabled.

data/c++.m4

@@ -185,6 +185,9 @@ m4_define([b4_public_types_declare],
       /// Default constructor.
       inline symbol_type ();
 
+      /// Destructive move, \a s is emptied.
+      inline void move (symbol_type& s);
+
       /// Constructor for tokens with semantic value.
       inline symbol_type (]b4_join([token_type t],
                                    [const semantic_type& v],
@@ -294,6 +297,16 @@ m4_define([b4_public_types_define],
   {
   }
 
+  inline
+  void
+  ]b4_parser_class_name[::symbol_type::move (symbol_type& s)
+  {
+    ]b4_variant_if([b4_symbol_variant([[s.type]], [value], [build], [s.value])],
+                   [value = s.value;])[
+    type = s.type;]b4_locations_if([
+    location = s.location;])[
+  }
+
   inline
   int
   ]b4_parser_class_name[::symbol_type::type_get_ () const