The container requirements say that for move assignment "All existing
elements of [the target] are either move assigned or destroyed". Some of
our containers currently use __make_move_if_noexcept which makes the
move depend on whether the element type is nothrow move constructible.
This is incorrect, because the standard says we must move assign, not
move or copy depending on the move constructor.

Use make_move_iterator instead so that we move unconditionally. This
ensures existing elements won't be copy assigned.

	PR libstdc++/92124
	* include/bits/forward_list.h
	(_M_move_assign(forward_list&&, false_type)): Do not use
	__make_move_if_noexcept, instead move unconditionally.
	* include/bits/stl_deque.h (_M_move_assign2(deque&&, false_type)):
	Likewise.
	* include/bits/stl_list.h (_M_move_assign(list&&, false_type)):
	Likewise.
	* include/bits/stl_vector.h (_M_move_assign(vector&&, false_type)):
	Likewise.
	* testsuite/23_containers/vector/92124.cc: New test.

From-SVN: r277113