libstdc++: Fix ranges::iter_move handling of rvalues [PR106612]

libstdc++: Fix ranges::iter_move handling of rvalues [PR106612]

The specification for std::ranges::iter_move apparently requires us to
handle types which do not satisfy std::indirectly_readable, for example
with overloaded operator* which behaves differently for different value
categories.

libstdc++-v3/ChangeLog:

	PR libstdc++/106612
	* include/bits/iterator_concepts.h (_IterMove::__iter_ref_t):
	New alias template.
	(_IterMove::__result): Use __iter_ref_t instead of
	std::iter_reference_t.
	(_IterMove::__type): Remove incorrect __dereferenceable
	constraint.
	(_IterMove::operator()): Likewise. Add correct constraints. Use
	__iter_ref_t instead of std::iter_reference_t. Forward parameter
	as correct value category.
	(iter_swap): Add comments.
	* testsuite/24_iterators/customization_points/iter_move.cc: Test
	that iter_move is found by ADL and that rvalue arguments are
	handled correctly.

Reviewed-by: Patrick Palka <ppalka@redhat.com>