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
