libstdc++: Fix std::ranges::iter_move for function references [PR119469]

libstdc++: Fix std::ranges::iter_move for function references [PR119469]

The result of std::move (or a cast to an rvalue reference) on a function
reference is always an lvalue. Because std::ranges::iter_move was using
the type std::remove_reference_t<X>&& as the result of std::move, it was
giving the wrong type for function references. Use a decltype-specifier
with declval<remove_reference_t<X>>() instead of just using the
remove_reference_t<X>&& type directly. This gives the right result,
while still avoiding the cost of doing overload resolution for
std::move.

libstdc++-v3/ChangeLog:

	PR libstdc++/119469
	* include/bits/iterator_concepts.h (_IterMove::__result): Use
	decltype-specifier instead of an explicit type.
	* testsuite/24_iterators/customization_points/iter_move.cc:
	Check results for function references.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>