libstdc++: Add [[nodiscard]] to <compare>

libstdc++: Add [[nodiscard]] to <compare>

This adds the [[nodiscard]] attribute to all conversion operators,
comparison operators, call operators and non-member functions in
<compare>. Nothing in this header except constructors has side effects.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* libsupc++/compare (partial_ordering, weak_ordering)
	(strong_ordering, is_eq, is_neq, is_lt, is_lteq, is_gt, is_gteq)
	(compare_three_way, strong_order, weak_order, partial_order)
	(compare_strong_order_fallback, compare_weak_order_fallback)
	(compare_partial_order_fallback, __detail::__synth3way): Add
	nodiscard attribute.
	* testsuite/18_support/comparisons/categories/zero_neg.cc: Add
	-Wno-unused-result to options.