Constrain swap overload for std::optional (LWG 2748)

	* doc/xml/manual/intro.xml: Document LWG 2748 status.
	* include/std/optional (optional<T>::swap): Use is_nothrow_swappable_v
	for exception specification.
	(swap(optional<T>&, optional<T>&)): Disable when T is not swappable.
	* testsuite/20_util/optional/swap/2.cc: New test.

From-SVN: r242415