Use locations in the variant example.

Yes, this obfuscates the point of this example, variants only.  But glr.cc
cannot work (yet?) without locations.  This change makes it easier to use
this example with glr.cc.

	* examples/variant.yy (assert): %define it.
	(locations): Request them.
	(yylex): Bind the location to the stage.

examples/variant.yy

@@ -1,8 +1,10 @@
 %debug
 %skeleton "lalr1.cc"
 %defines
+%define assert
 %define variant
 %define lex_symbol
+%locations
 
 %code requires // *.hh
 {
@@ -85,26 +87,28 @@ yy::parser::symbol_type
 yylex ()
 {
   static int stage = -1;
-  switch (++stage)
+  ++stage;
+  yy::parser::location_type loc(0, stage + 1, stage + 1);
+  switch (stage)
   {
     case 0:
-      return yy::parser::make_TEXT ("I have three numbers for you.");
+      return yy::parser::make_TEXT ("I have three numbers for you.", loc);
     case 1:
     case 2:
     case 3:
-      return yy::parser::make_NUMBER (stage);
+      return yy::parser::make_NUMBER (stage, loc);
     case 4:
-      return yy::parser::make_TEXT ("And that's all!");
+      return yy::parser::make_TEXT ("And that's all!", loc);
     default:
-      return yy::parser::make_END_OF_FILE ();
+      return yy::parser::make_END_OF_FILE (loc);
   }
 }
 
 // Mandatory error function
 void
-yy::parser::error (const std::string& message)
+yy::parser::error (const yy::parser::location_type& loc, const std::string& msg)
 {
-  std::cerr << message << std::endl;
+  std::cerr << loc << ": " << msg << std::endl;
 }
 
 int