This adds the missing parts of P0896R4 to reverse_iterator and
move_iterator, so that they meet the C++20 requirements. This should be
the last piece of P0896R4, meaning ranges support is now complete.

The PR 94354 bug with reverse_iterator's comparisons is fixed for C++20
only, but that change should be extended to C++11, C++14 and C++17 modes
in stage 1.

	* include/bits/stl_iterator.h (reverse_iterator::iterator_concept)
	(reverse_iterator::iterator_category): Define for C++20.
	(reverse_iterator): Define comparison operators correctly for C++20.
	(__normal_iterator): Add constraints to comparison operators for C++20.
	(move_iterator::operator++(int)) [__cpp_lib_concepts]: Define new
	overload for input iterators.
	(move_iterator): Add constraints to comparison operators for C++20.
	Define operator<=> for C++20.
	* testsuite/24_iterators/move_iterator/input_iterator.cc: New test.
	* testsuite/24_iterators/move_iterator/move_only.cc: New test.
	* testsuite/24_iterators/move_iterator/rel_ops_c++20.cc: New test.
	* testsuite/24_iterators/reverse_iterator/rel_ops_c++20.cc: New test.