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.