libstdc++: Replace deduced return type in ranges::iter_move (PR 92894)

libstdc++: Replace deduced return type in ranges::iter_move (PR 92894)

The deduced return type causes the instantiation of the function body,
which can then require the instantiation of std::projected::operator*
which is intentionally not defined.

This patch uses a helper trait to define the return type, so that the
function body doesn't need to be instantiated. That helper trait can
then also be used in other places that currently check the return type
of ranges::iter_move (iter_rvalue_reference_t and indirectly_readable).

2020-05-01  Jonathan Wakely  <jwakely@redhat.com>
	    Patrick Palka  <ppalka@redhat.com>

	PR libstdc++/92894
	* include/bits/iterator_concepts.h (ranges::__cust_imove::_IMove):
	Add trait to determine return type and an alias for it.
	(ranges::__cust_imove::_IMove::operator()): Use __result instead of
	deduced return type.
	(iter_rvalue_reference_t): Use _IMove::__type instead of checking
	the result of ranges::iter_move.
	(__detail::__indirectly_readable_impl): Use iter_rvalue_reference_t
	instead of checking the result of ranges::iter_move.
	* testsuite/24_iterators/customization_points/92894.cc: New test.
	* testsuite/24_iterators/indirect_callable/92894.cc: New test.