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

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.