libstdc++: Tweak dg-error patterns for C++26 constexpr changes

libstdc++: Tweak dg-error patterns for C++26 constexpr changes

libstdc++-v3/ChangeLog:

	* testsuite/25_algorithms/copy/debug/constexpr_neg.cc:
	* testsuite/25_algorithms/copy_backward/debug/constexpr_neg.cc:
	* testsuite/25_algorithms/equal/debug/constexpr_neg.cc:
	* testsuite/25_algorithms/lower_bound/debug/constexpr_partitioned_neg.cc:
	* testsuite/25_algorithms/lower_bound/debug/constexpr_partitioned_pred_neg.cc:
	* testsuite/25_algorithms/lower_bound/debug/constexpr_valid_range_neg.cc:
	* testsuite/25_algorithms/upper_bound/debug/constexpr_partitioned_neg.cc:
	* testsuite/25_algorithms/upper_bound/debug/constexpr_partitioned_pred_neg.cc:
	* testsuite/25_algorithms/upper_bound/debug/constexpr_valid_range_neg.cc: