libstdc++: Fix bug in iterator_traits<common_iterator<S,I>>

libstdc++: Fix bug in iterator_traits<common_iterator<S,I>>

The wrong type was being used in the __common_iter_has_arrow constraint,
creating a circular dependency where the iterator_traits specialization
was needed before it was complete. The correct parameter for the
__common_iter_has_arrow concept is the first template argument of  the
common_iterator, not the common_iterator itself.

	* include/bits/stl_iterator.h (__detail::__common_iter_ptr): Change
	to take parameters of common_iterator, instead of the common_iterator
	type itself. Fix argument for __common_iter_has_arrow constraint.
	(iterator_traits<common_iterator<I, S>>::pointer): Adjust.