Implement LWG 2451, optional<T> should 'forward' T's
	implicit conversions.
	* include/experimental/optional (__is_optional_impl, __is_optional):
	New.
	(optional()): Make constexpr and default.
	(optional(_Up&&), optional(const optional<_Up>&),
	optional(optional<_Up>&& __t): New.
	(operator=(_Up&&)): Constrain.
	(operator=(const optional<_Up>&), operator=(optional<_Up>&&)): New.
	* testsuite/experimental/optional/cons/value.cc:
	Add tests for the functionality added by LWG 2451.
	* testsuite/experimental/optional/cons/value_neg.cc: New.

From-SVN: r238049