qiterator.h forward declares iterator categories

qiterator.h has the following piece of code:

namespace std {
    struct bidirectional_iterator_tag;
    struct random_access_iterator_tag;
}

This breaks compilation with libc++ because libc++ has these in std::__1 (currently, might change in future as its version is bumped). Forward declaring things in std namespace is forbidden by the standard, so that piece of code should not have been written. It should be replaced by #include <iterator>.

Here is a minimal example that is broken by this bug:

#include <iterator>
#include <QObject>

class A {};

template<typename T>
class X
{
public:
    class iterator {
    public:
        typedef std::random_access_iterator_tag  iterator_category;
        typedef ptrdiff_t difference_type;
        typedef T value_type;
        typedef T *pointer;
        typedef T &reference;
    };  
    
};

void t()
{
  std::iterator_traits<X<A*>::iterator>::iterator_category x;
}

test.cc:23:42: error: no member named 'iterator_category' in 'std::__1::iterator_traits<X<A *>::iterator>'
std::iterator_traits<X<A*>::iterator>::iterator_category x;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
1 error generated.

This is probably the root cause of QTBUG-25922.