[libc++] [ranges] Uncomment operator<=> in transform and iota iterators.

[libc++] [ranges] Uncomment operator<=> in transform and iota iterators.

The existing tests for transform_view::iterator weren't quite right,
and can be simplified now that we have more of C++20 available to us.
Having done that, let's use the same pattern for iota_view::iterator
as well.

Differential Revision: https://reviews.llvm.org/D110774