libstdc++: Also move the [[nodiscard]] attributes in <compare>

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

libstdc++-v3/ChangeLog:

	* libsupc++/compare (compare_three_way, strong_order)
	(weak_order, partial_order, compare_strong_order_fallback)
	(compare_weak_order_fallback, compare_partial_order_fallback):
	Move nodiscard attributes to correct location.