Fix Debug mode Undefined Behavior

[François Dumont]

After the fix of PR 91910 I tried to consider other possible race 
condition and I think we still have a problem.

Like stated in the PR when a container is destroyed all associated 
iterators are made singular. If at the same time another thread try to 
access this iterator the _M_singular check will face a data race when 
accessing _M_sequence member. In case of race condition the program is 
likely to abort but maybe because of memory access violation rather than 
a clear singular iterator assertion.

To avoid this I rework _M_sequence manipulation to use atomic read when 
necessary and make sure that otherwise container mutex is locked.

             * src/c++/debug.cc
             (_Safe_sequence_base::_M_attach_single): Set attached iterator
             sequence pointer and version.
             (_Safe_sequence_base::_M_detach_single): Reset detached 
iterator.
             (_Safe_iterator_base::_M_attach): Remove attached iterator 
sequence
             pointer and version assignments.
             (_Safe_iterator_base::_M_attach_single): Likewise.
             (_Safe_iterator_base::_M_detach_single): Remove detached 
iterator
             reset.
             (_Safe_iterator_base::_M_singular): Use atomic load to 
access parent
             sequence.
             (_Safe_iterator_base::_M_can_compare): Likewise.
             (_Safe_iterator_base::_M_get_mutex): Likewise.
             (_Safe_local_iterator_base::_M_attach): Remove attached 
iterator container
             pointer and version assignments.
             (_Safe_local_iterator_base::_M_attach_single): Likewise.
(_Safe_unordered_container_base::_M_attach_local_single):
             Set attached iterator container pointer and version.
(_Safe_unordered_container_base::_M_detach_local_single): Reset detached
             iterator.

Running tests in Debug mode.

Ok to commit if successful ?

François

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