libstdc++: Simplify constraints on std::compare_three_way

libstdc++: Simplify constraints on std::compare_three_way

The __3way_builtin_ptr_cmp concept can use three_way_comparable_with to
check whether <=> is valid. Doing that makes it obvious that the
disjunction on compare_three_way::operator() is redundant, because
the second constraint subsumes the first.

The workaround for PR c++/91073 can also be removed as that bug is fixed
now.

	* libsupc++/compare (__detail::__3way_builtin_ptr_cmp): Use
	three_way_comparable_with.
	(__detail::__3way_cmp_with): Remove workaround for fixed bug.
	(compare_three_way::operator()): Remove redundant constraint from
	requires-clause.
	(__detail::_Synth3way::operator()): Use three_way_comparable_with
	instead of workaround.
	* testsuite/18_support/comparisons/object/93479.cc: Prune extra
	output due to simplified constraints on compare_three_way::operator().