This is a trivial fix following a recent change on __glibcxx_assert.

     libstdc++: Fix dg-prune-output assertion message

     Since __glibcxx_assert changes in r6b42b5a the generated assertion 
message
     has changed.

     libstdc++-v3/ChangeLog:

             * testsuite/25_algorithms/copy/debug/constexpr_neg.cc: 
Replace 'failed_assertion'
             dg-prune-output reason with 'builtin_unreachable'.
             * 
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.

François