[libc++] Fix signatures of counted_iterator::base(), transform_view::iterator::base().

[• Quuxplusone]

Summary

Drive-by stop calling .base() on test iterators in the test, and improve the transform_view::iterator/sentinel tests.

[ldionne]

LGTM, but I think we suffer from the same problem in other iterators, for example transform_view::iterator but probably others too. Any appetite for fixing all of them in this PR?

[• Quuxplusone]

Turns out that transform_view::iterator is the only other one, that we've already implemented, that doesn't require major other changes for C++20.

move_iterator and reverse_iterator are also affected but require major changes for C++20 that we haven't implemented yet.
filter_view::iterator is affected but we haven't implemented it yet.
lazy_split_view::iterator is affected but we haven't implemented it yet.
elements_view::iterator is affected but we haven't implemented it yet.

[• Quuxplusone]

Fix a test failure, and improve the transform_view::{iterator/sentinel} tests.

[• Quuxplusone]

Rebase to poke CI.

[• Quuxplusone]

Turns out this is LWG3533; so now we can mark that issue Complete.
CI is still green.

[Mordante]

In D117329#3263611, @Quuxplusone wrote:

Turns out this is LWG3533; so now we can mark that issue Complete.

Please add this to the commit message and mention [range.elements.iterator] has been omitted since we haven't implemented that yet.
(I expect the person implementing that to use the current wording in the Standard.)

LTGM!