We already have conditional noexcept so this just constrains the
non-member swap overload.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/experimental/propagate_const (swap): Constrain.
	* testsuite/experimental/propagate_const/swap/lwg3413.cc: New test.

Tested x86_64-linux. Committed to trunk.