libstdc++: Implement P2770R0 changes to join_view / join_with_view

This C++23 paper fixes an issue in these views when adapting a certain
kind of non-forward range, and we treat it as a DR against C++20.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/bits/regex.h (regex_iterator::iterator_concept):
	Define for C++20 as per P2770R0.
	(regex_token_iterator::iterator_concept): Likewise.
	* include/std/ranges (__detail::__as_lvalue): Define.
	(join_view::_Iterator): Befriend join_view.
	(join_view::_Iterator::_M_satisfy): Use _M_get_outer
	instead of _M_outer.
	(join_view::_Iterator::_M_get_outer): Define.
	(join_view::_Iterator::_Iterator): Split constructor taking
	_Parent argument into two as per P2770R0.  Remove constraint on
	default constructor.
	(join_view::_Iterator::_M_outer): Make this data member present
	only when the underlying range is forward.
	(join_view::_Iterator::operator++): Use _M_get_outer instead of
	_M_outer.
	(join_view::_Iterator::operator--): Use __as_lvalue helper.
	(join_view::_Iterator::operator==): Adjust constraints as per
	P2770R0.
	(join_view::_Sentinel::__equal): Use _M_get_outer instead of
	_M_outer.
	(join_view::_M_outer): New data member when the underlying range
	is non-forward.
	(join_view::begin): Adjust definition as per P2770R0.
	(join_view::end): Likewise.
	(join_with_view::_M_outer_it): New data member when the
	underlying range is non-forward.
	(join_with_view::begin): Adjust definition as per P2770R0.
	(join_with_view::end): Likewise.
	(join_with_view::_Iterator::_M_outer_it): Make this data member
	present only when the underlying range is forward.
	(join_with_view::_Iterator::_M_get_outer): Define.
	(join_with_view::_Iterator::_Iterator): Split constructor
	taking _Parent argument into two as per P2770R0.  Remove
	constraint on default constructor.
	(join_with_view::_Iterator::_M_update_inner): Adjust definition
	as per P2770R0.
	(join_with_view::_Iterator::_M_get_inner): Likewise.
	(join_with_view::_Iterator::_M_satisfy): Adjust calls to
	_M_get_inner.  Use _M_get_outer instead of _M_outer_it.
	(join_with_view::_Iterator::operator==): Adjust constraints
	as per P2770R0.
	(join_with_view::_Sentinel::operator==): Use _M_get_outer
	instead of _M_outer_it.
	* testsuite/std/ranges/adaptors/p2770r0.cc: New test.