Ok, 3 major changes for debug mode in one commit:

1.  I had been detecting and trapping iterator == and \!= among iterators
    in different containers as an error.  But the trapping itself is actually
    an error.

    Consider:

    #include <iostream>
    #include <vector>
    #include <algorithm>

    template <class C>
    void
    display(const C& c)
    {
        std::cout << "{";
        bool first = true;
        for (const auto& x : c)
        {
            if (\!first)
                std::cout << ", ";
            first = false;
            std::cout << x;
        }
        std::cout << "}\n";
    }

    int
    main()
    {
        typedef std::vector<int> V;
        V v1 = {1, 3, 5};
        V v2 = {2, 4, 6};
        display(v1);
        display(v2);
        V::iterator i = std::find(v1.begin(), v1.end(), 1);
        V::iterator j = std::find(v2.begin(), v2.end(), 2);
        if (*i == *j)
            i = j;    // perfectly legal
        // ...
        if (i \!= j)   // the only way to check
            v2.push_back(*i);
        display(v1);
        display(v2);
    }

    It is legal to assign an iterator from one container to another of the
    same type.  This is required to work.  One might want to test whether or
    not such an assignment had been made.  The way one performs such a check
    is using the iterator's ==, \!= operator.  This is a logical and necessary
    function and does not constitute an error.

2.  I had a header circular dependence bug when _LIBCPP_DEBUG2 is defined.
    This caused a problem in several of the libc++ tests.
    Fixed.

3.  There is a serious problem when _LIBCPP_DEBUG2=1 at the moment in that
    std::basic_string is inoperable.  std::basic_string uses __wrap_iterator
    to implement its iterators.  __wrap_iterator has been rigged up in debug
    mode to support vector.  But string hasn't been rigged up yet.  This means
    that one gets false positives when using std::string in debug mode.  I've
    upped std::string's priority in www/debug_mode.html.