libstdc++: Update expected error for debug/constexpr*_neg.cc tests

We no longer hit a __builtin_unreachable() in these tests, so we need to
update the dg-error patterns to match _Error_formatter::_M_error().

We can also remove some dg-prune-output directives matching notes saying
"in 'constexpr' expansion" because that's done globally in prune.exp.

libstdc++-v3/ChangeLog:

	* testsuite/25_algorithms/copy/debug/constexpr_neg.cc: Adjust
	dg-error pattern.
	* testsuite/25_algorithms/copy_backward/debug/constexpr_neg.cc:
	Likewise.
	* testsuite/25_algorithms/equal/debug/constexpr_neg.cc:
	Likewise.
	* testsuite/25_algorithms/lower_bound/debug/constexpr_partitioned_neg.cc:
	Likewise.
	* testsuite/25_algorithms/lower_bound/debug/constexpr_partitioned_pred_neg.cc:
	Likewise.
	* testsuite/25_algorithms/lower_bound/debug/constexpr_valid_range_neg.cc:
	Likewise.
	* testsuite/25_algorithms/upper_bound/debug/constexpr_partitioned_neg.cc:
	Likewise.
	* testsuite/25_algorithms/upper_bound/debug/constexpr_partitioned_pred_neg.cc:
	Likewise.
	* testsuite/25_algorithms/upper_bound/debug/constexpr_valid_range_neg.cc:
	Likewise.