libstdc++: Fix _OutputIteratorConcept checks in algorithms

libstdc++: Fix _OutputIteratorConcept checks in algorithms

The _OutputIteratorConcept should be checked using the correct value
category. The std::move_backward and std::copy_backward algorithms
should use _OutputIteratorConcept instead of _ConvertibleConcept.

In order to use the correct value category, the concept should use a
function that returns _ValueT instead of using an lvalue data member.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/boost_concept_check.h (_OutputIteratorConcept):
	Use a function to preserve value category of the type.
	* include/bits/stl_algobase.h (copy, move, fill_n): Use a
	reference as the second argument for _OutputIteratorConcept.
	(copy_backward, move_backward): Use _OutputIteratorConcept
	instead of _ConvertibleConcept.