libstdc++: Implement LWG 3569 changes to join_view::_Iterator
libstdc++-v3/ChangeLog:
* include/std/ranges (join_view::_Iterator::_M_satisfy):
Adjust resetting _M_inner as per LWG 3569.
(join_view::_Iterator::_M_inner): Wrap in std::optional
as per LWG 3569.
(join_view::_Iterator::_Iterator): Relax constraints as
per LWG 3569.
(join_view::_Iterator::operator*): Adjust as per LWG 3569.
(join_view::_Iterator::operator->): Likewise.
(join_view::_Iterator::operator++): Likewise.
(join_view::_Iterator::operator--): Likewise.
(join_view::_Iterator::iter_move): Likewise.
(join_view::_Iterator::iter_swap): Likewise.
* testsuite/std/ranges/adaptors/join.cc (test14): New test.