Use locations in the variant example.

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.