[libc++] Fix LWG3533 "Make `base() const&` consistent..."

Fixed in counted_iterator and transform_view::iterator.
The LWG issue also affected elements_view::iterator, but we haven't
implemented that one yet, and whoever does implement it will get
the fix for free if they just follow the working draft's wording.

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

Differential Revision: https://reviews.llvm.org/D117329