libsupc++: Implement comparison algorithms for C++20

libsupc++: Implement comparison algorithms for C++20

This is incomplete because std::strong_order doesn't support
floating-point types.

The partial_order and weak_order tests use VERIFY instead of
static_assert because of PR 92431.

	* libsupc++/compare (strong_order, weak_order, partial_order)
	(compare_strong_order_fallback, compare_weak_order_fallback)
	(compare_partial_order_fallback): Define customization point objects
	for C++20.
	* testsuite/18_support/comparisons/algorithms/partial_order.cc: New
	test.
	* testsuite/18_support/comparisons/algorithms/strong_order.cc: New
	test.
	* testsuite/18_support/comparisons/algorithms/weak_order.cc: New test.

From-SVN: r278149