libstdc++: Remove workarounds for constrained nested class templates

libstdc++: Remove workarounds for constrained nested class templates

With PR c++/92078 and PR c++/92103 both fixed, nested class templates
can now be constrained. That means a number of namespace-scope helpers
can be moved to the class scope, so they're only visible where they're
needed.

	* include/bits/iterator_concepts.h (__detail::__ptr, __detail::__ref)
	(__detail::__cat, __detail::__diff): Move to class scope in the
	relevant __iterator_traits specializations.
	(__iterator_traits<>): Use nested class templates instead of ones from
	namespace __detail.
	* include/bits/stl_iterator.h (__detail::__common_iter_ptr): Move to
	class scope in iterator_traits<common_iterator<I, S>>.
	(iterator_traits<common_iterator<I, S>>): Use nested class template
	instead of __detail::__common_iter_ptr.