libstdc++: Implement P2259R1 changes [PR95983]

libstdc++: Implement P2259R1 changes [PR95983]

This implements the wording changes of P2259R1 "Repairing input range
adaptors and counted_iterator", which resolves LWG 3283, 3289 and 3408.

The wording changes are relatively straightforward, but they require
some boilerplate to implement: the changes to make a type alias
"conditionally present" in some iterator class are realized by defining
a base class template and an appropriately constrained partial
specialization thereof that contains the type alias, and having the
iterator class derive from this base class.  Sometimes the relevant
condition depend on members from the iterator class, but because a
class is incomplete when instantiating its bases, the constraints on
the partial specialization can't use anything from the iterator class.
This patch works around this by hoisting these members out to the
enclosing scope (e.g. transform_view::_Iterator::_Base is hoisted out
to transform_view::_Base so that transform_view::__iter_cat can use it).

This patch also implements the proposed resolution of LWG 3291 to rename
iota_view::iterator_category to iota_view::iterator_concept, which was
previously problematic due to LWG 3408.

libstdc++-v3/ChangeLog:

	PR libstdc++/95983
	* include/bits/stl_iterator.h (__detail::__move_iter_cat):
	Define.
	(move_iterator): Derive from the above in C++20 in order to
	conditionally define iterator_category as per P2259.
	(move_iterator::__base_cat): No longer used, so remove.
	(move_iterator::iterator_category): Remove in C++20.
	(__detail::__common_iter_use_postfix_proxy): Define.
	(common_iterator::_Proxy): Rename to ...
	(common_iterator:__arrow_proxy): ... this.
	(common_iterator::__postfix_proxy): Define as per P2259.
	(common_iterator::operator->): Adjust.
	(common_iterator::operator++): Adjust as per P2259.
	(iterator_traits<common_iterator>::_S_iter_cat): Define.
	(iterator_traits<common_iterator>::iterator_category): Change as
	per P2259.
	(__detail::__counted_iter_value_type): Define.
	(__detail::__counted_iter_concept): Define.
	(__detail::__counted_iter_cat): Define.
	(counted_iterator): Derive from the above three classes in order
	to conditionally define value_type, iterator_concept and
	iterator category respectively as per P2259.
	(counted_iterator::operator->): Define as per P2259.
	(incrementable_traits<counted_iterator>): Remove as per P2259.
	(iterator_traits<counted_iterator>): Adjust as per P2259.
	* include/std/ranges (__detail::__iota_view_iter_cat): Define.
	(iota_view::_Iterator): Derive from the above in order to
	conditionally define iterator_category as per P2259.
	(iota_view::_S_iter_cat): Rename to ...
	(iota_view::_S_iter_concept): ... this.
	(iota_view::iterator_concept): Use it to apply LWG 3291 changes.
	(iota_view::iterator_category): Remove.
	(__detail::__filter_view_iter_cat): Define.
	(filter_view::_Iterator): Derive from the above in order to
	conditionally define iterator_category as per P2259.
	(filter_view::_Iterator): Move to struct __filter_view_iter_cat.
	(filter_view::_Iterator::iterator_category): Remove.
	(transform_view::_Base): Define.
	(transform_view::__iter_cat): Define.
	(transform_view::_Iterator): Derive from the above in order to
	conditionally define iterator_category as per P2259.
	(transform_view::_Iterator::_Base): Just alias
	transform_view::_Base.
	(transform_view::_Iterator::_S_iter_cat): Move to struct
	transform_view::__iter_cat.
	(transform_view::_Iterator::iterator_category): Remove.
	(transform_view::_Sentinel::_Base): Just alias
	transform_view::_Base.
	(join_view::_Base): Define.
	(join_view::_Outer_iter): Define.
	(join_view::_Inner_iter): Define.
	(join_view::_S_ref_is_glvalue): Define.
	(join_view::__iter_cat): Define.
	(join_view::_Iterator): Derive from it in order to conditionally
	define iterator_category as per P2259.
	(join_view::_Iterator::_Base): Just alias join_view::_Base.
	(join_view::_Iterator::_S_ref_is_glvalue): Just alias
	join_view::_S_ref_is_glvalue.
	(join_view::_Iterator::_S_iter_cat): Move to struct
	transform_view::__iter_cat.
	(join_view::_Iterator::_Outer_iter): Just alias
	join_view::_Outer_iter.
	(join_view::_Iterator::_Inner_iter): Just alias
	join_view::_Inner_iter.
	(join_view::_Iterator::iterator_category): Remove.
	(join_view::_Sentinel::_Base): Just alias join_view::_Base.
	(__detail::__split_view_outer_iter_cat): Define.
	(__detail::__split_view_inner_iter_cat): Define.
	(split_view::_Base): Define.
	(split_view::_Outer_iter): Derive from __split_view_outer_iter_cat
	in order to conditionally define iterator_category as per P2259.
	(split_view::_Outer_iter::iterator_category): Remove.
	(split_view::_Inner_iter): Derive from __split_view_inner_iter_cat
	in order to conditionally define iterator_category as per P2259.
	(split_view::_Inner_iter::_S_iter_cat): Move to
	__split_view_inner_iter_cat.
	(split_view::_Inner_iter::iterator_category): Remove.
	(elements_view::_Base): Define.
	(elements_view::__iter_cat): Define.
	(elements_view::_Iterator): Derive from the above in order to
	conditionall define iterator_category as per P2259.
	(elements_view::_Iterator::_Base): Just alias
	elements_view::_Base.
	(elements_view::_Iterator::_S_iter_concept)
	(elements_view::_Iterator::iterator_concept): Define as per
	P2259.
	(elements_view::_Iterator::iterator_category): Remove.
	(elements_view::_Sentinel::_Base): Just alias
	elements_view::_Base.
	* testsuite/24_iterators/headers/iterator/synopsis_c++20.cc:
	Adjust constraints on iterator_traits<counted_iterator>.
	* testsuite/std/ranges/p2259.cc: New test.