[libc++] Refactor stride_counting_iterator

Instead of storing the wrapped iterator inside the stride_counting_iterator,
store its base so we can have e.g. a stride_counting_iterator of an
input_iterator (which was previously impossible because input_iterators
are not copyable). Also a few other simplifications in stride_counting_iterator.

As a fly-by fix, remove the member base() functions, which are super
confusing.

Differential Revision: https://reviews.llvm.org/D116613