libstdc++: Fix test that fails for C++20

Also restore the test for 'a < a' that was removed by r12-2537 because
it is ill-formed. We still want to test operator< for tuple, we just
need to not use std::nullptr_t in that tuple type.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/tuple/comparison_operators/overloaded.cc:
	Restore test for operator<.
	* testsuite/20_util/tuple/comparison_operators/overloaded2.cc:
	Adjust expected errors for C++20.

libstdc++-v3/testsuite/20_util/tuple/comparison_operators/overloaded.cc

@@ -48,3 +48,9 @@ TwistedLogic operator<(const Compares&, const Compares&) { return {false}; }
 
 auto a = std::make_tuple(nullptr, Compares{}, 2, 'U');
 auto b = a == a;
+
+#if ! __cpp_lib_three_way_comparison
+// Not valid in C++20, because TwistedLogic doesn't model boolean-testable.
+auto c = std::make_tuple("", Compares{}, 2, 'U');
+auto d = c < c;
+#endif

libstdc++-v3/testsuite/20_util/tuple/comparison_operators/overloaded2.cc

@@ -49,5 +49,7 @@ TwistedLogic operator<(const Compares&, const Compares&) { return {false}; }
 auto a = std::make_tuple(nullptr, Compares{}, 2, 'U');
 auto b = a < a;
 
-// { dg-error "ordered comparison" "" { target *-*-* } 0 }
+// { dg-error "no match for 'operator<'" "" { target c++20 } 0 }
+// { dg-error "no match for .*_Synth3way|in requirements" "" { target c++20 } 0 }
+// { dg-error "ordered comparison" "" { target c++17_down } 0 }
 // { dg-error "not a return-statement" "" { target c++11_only } 0 }