libstdc++: Constrain equality ops for std::pair, std::tuple, std::variant

Implement the changes from P2944R3 which add constraints to the
comparison operators of std::pair, std::tuple, and std::variant.

The paper also changes std::optional, but we already constrain its
comparisons using SFINAE on the return type. However, we need some
additional constraints on the [optional.comp.with.t] operators that
compare an optional with a value. The paper doesn't say to do that, but
I think it's needed because otherwise when the comparison for two
optional objects fails its constraints, the two overloads that are
supposed to be for comparing to a non-optional become the best overload
candidates, but are ambiguous (and we don't even get as far as checking
the constraints for satisfaction). I reported LWG 4072 for this.

The paper does not change std::expected, but probably should have done.
I'll submit an LWG issue about that and implement it separately.

Also add [[nodiscard]] to all these comparison operators.

libstdc++-v3/ChangeLog:

	* include/bits/stl_pair.h (operator==): Add constraint.
	* include/bits/version.def (constrained_equality): Define.
	* include/bits/version.h: Regenerate.
	* include/std/optional: Define feature test macro.
	(__optional_rep_op_t): Use is_convertible_v instead of
	is_convertible.
	* include/std/tuple: Define feature test macro.
	(operator==, __tuple_cmp, operator<=>): Reimplement C++20
	comparisons using lambdas. Add constraints.
	* include/std/utility: Define feature test macro.
	* include/std/variant: Define feature test macro.
	(_VARIANT_RELATION_FUNCTION_TEMPLATE): Add constraints.
	(variant): Remove unnecessary friend declarations for comparison
	operators.
	* testsuite/20_util/optional/relops/constrained.cc: New test.
	* testsuite/20_util/pair/comparison_operators/constrained.cc:
	New test.
	* testsuite/20_util/tuple/comparison_operators/constrained.cc:
	New test.
	* testsuite/20_util/variant/relops/constrained.cc: New test.
	* testsuite/20_util/tuple/comparison_operators/overloaded.cc:
	Disable for C++20 and later.
	* testsuite/20_util/tuple/comparison_operators/overloaded2.cc:
	Remove dg-error line for target c++20.

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