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.

Unlike on the master branch, this backport to gcc-10 does not change the
iter_rvalue_reference_t alias template and __indirectly_readable_impl
concept to use the new trait.

Backport from mainline
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.
	* testsuite/24_iterators/customization_points/92894.cc: New test.
	* testsuite/24_iterators/indirect_callable/92894.cc: New test.

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