This implements most of the remaining C++20 additions to the <iterator>
header.

	* include/bits/iterator_concepts.h (ranges::iter_swap): Fix parameter
	types of poison pill overload. Use remove_reference_t when checking
	constraints.
	* include/bits/stl_iterator.h (move_sentinel): Define for C++20.
	(move_iterator): Adjust definitions of nested types for C++20. Add
	hidden friends for move_sentinel operations, iter_move and iter_swap.
	(common_iterator, counted_iterator): Define for C++20.
	* testsuite/24_iterators/move_iterator/cust.cc: New test.
	* testsuite/24_iterators/move_iterator/sentinel.cc: New test.
	* testsuite/24_iterators/common_iterator/1.cc: New test.
	* testsuite/24_iterators/counted_iterator/1.cc: New test.

From-SVN: r278698