libstdc++: Fix ranges::move and ranges::move_backward to use iter_move [PR105609]

libstdc++: Fix ranges::move and ranges::move_backward to use iter_move [PR105609]

The ranges::move and ranges::move_backward algorithms are supposed to
use ranges::iter_move(iter) instead of std::move(*iter), which matters
for an iterator type with an iter_move overload findable by ADL.

Currently those algorithms use std::__assign_one which uses std::move,
so define a new ranges::__detail::__assign_one helper function that uses
ranges::iter_move.

libstdc++-v3/ChangeLog:

	PR libstdc++/105609
	* include/bits/ranges_algobase.h (__detail::__assign_one): New
	helper function.
	(__copy_or_move, __copy_or_move_backward): Use new function
	instead of std::__assign_one.
	* testsuite/25_algorithms/move/constrained.cc: Check that
	ADL iter_move is used in preference to std::move.
	* testsuite/25_algorithms/move_backward/constrained.cc:
	Likewise.