libstdc++: Implement P2520R0 changes to move_iterator's iterator_concept

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (move_iterator::_S_iter_concept):
	Define.
	(__cpp_lib_move_iterator_concept): Define for C++20.
	(move_iterator::iterator_concept): Strengthen as per P2520R0.
	* include/std/version (__cpp_lib_move_iterator_concept): Define
	for C++20.
	* testsuite/24_iterators/move_iterator/p2520r0.cc: New test.