* include/std/optional (_Optional_payload): Use variable templates
	for conditions in default template arguments and exception
	specifications.
	(optional): Likewise. Adjust indentation.
	(optional::__not_self, optional::__not_tag, optional::_Requires): New
	SFINAE helpers.
	(optional::optional): Use new helpers in constructor constraints.
	* include/std/type_traits (__or_v, __and_v): New variable templates.
	* testsuite/20_util/optional/cons/value_neg.cc: Change dg-error to
	dg-prune-output. Remove unused header.

From-SVN: r263657