libstdc++: Define <=> for tuple, optional and variant

libstdc++: Define <=> for tuple, optional and variant

Another piece of P1614R2.

	* include/std/optional (operator<=>(optional<T>, optional<U>))
	(operator<=>(optional<T>, nullopt), operator<=>(optional<T>, U)):
	Define for C++20.
	* include/std/tuple (__tuple_cmp): New helper function for <=>.
	(operator<=>(tuple<T...>, tuple<U>...)): Define for C++20.
	* include/std/variant (operator<=>(variant<T...>, variant<T...>))
	(operator<=>(monostate, monostate)): Define for C++20.
	* testsuite/20_util/optional/relops/three_way.cc: New test.
	* testsuite/20_util/tuple/comparison_operators/three_way.cc: New test.
	* testsuite/20_util/variant/89851.cc: Move to ...
	* testsuite/20_util/variant/relops/89851.cc: ... here.
	* testsuite/20_util/variant/90008.cc: Move to ...
	* testsuite/20_util/variant/relops/90008.cc: ... here.
	* testsuite/20_util/variant/relops/three_way.cc: New test.