[libc++][NFC] Make data members of counted_iterator private (#70491)

There's no reason for them to be public AFAICT. I came across this while
auditing the code bases for places where we'd be using
[[no_unique_address]] incorrectly. Since one of these members is marked
as [[no_unique_address]], this is an extra reason to keep this private
to ensure the member isn't escaped anywhere.