libstdc++: Implement LWG 3391 changes to move/counted_iterator::base()

libstdc++: Implement LWG 3391 changes to move/counted_iterator::base()

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (move_iterator::base): Make the
	const& overload unconstrained and return a const reference as
	per LWG 3391.  Make unconditionally noexcept.
	(counted_iterator::base): Likewise.
	* testsuite/24_iterators/move_iterator/lwg3391.cc: New test.
	* testsuite/24_iterators/move_iterator/move_only.cc: Adjust
	has_member_base concept to decay-copy the result of base().