Dumpers for some std container iterators don't work with _GLIBCXX_DEBUG

Qt Creator's debugging helpers (dumpers) don't work for (some) iterators for containers from the C++ standard library when using libstdc++ and _GLIBCXX_DEBUG. See QTCREATORBUG-20476 (which was about dumpers for containers themselves, not their iterators) for more background.

This can be seen e.g. by running Qt Creator's own dumper tests with a corresponding environment variable set:

QTC_USE_GLIBCXXDEBUG_FOR_TEST=1 ctest -V -R tst_debugger_dumpers 

This causes e.g. the StdSet and StdMap dumpers to fail. These tests succeed when running without QTC_USE_GLIBCXXDEBUG_FOR_TEST=1 set.

Quoting from ansemkiv 's comment https://bugreports.qt.io/browse/QTCREATORBUG-20476?focusedId=834100&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-834100 :

enabling QTC_USE_GLIBCXXDEBUG_FOR_TEST globally is known to fail. Besides the two tests that you mentioned, there are also additional failures in QJson and QCbor tests, if I remember correctly. On the other hand, my fix is only limited to container dumpers itself, whereas StdSet and StdMap tests actually fail to dump iterator values for these containers. I had a branch that fixes that, but it did not quite work for LLDB on Windows, and the scope of this bug had been covered, so it hasn't been merged. I could come back to it and have a look again if you create a new ticket, feel free to do it.