libstdc++: Add missing P0896 changes to <iterator>

libstdc++: Add missing P0896 changes to <iterator>

I noticed that the following changes from this paper were not yet
implemented.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (reverse_iterator::iter_move):
	Define for C++20 as per P0896.
	(reverse_iterator::iter_swap): Likewise.
	(move_iterator::operator*): Apply P0896 changes for C++20.
	(move_iterator::operator[]): Likewise.
	* testsuite/24_iterators/reverse_iterator/cust.cc: New test.

(cherry picked from commit 080a23bce12fa3f25860631f019777f728d2ef11)