[libc++][ranges] Implement Ranges changes to `istream{,buf}_iterator`.

The changes from the One Ranges Proposal amount to adding:
- a constructor that takes a `default_sentinel_t` and is equivalent to
  the default constructor;
- an `operator==` that compares the iterator to `default_sentinel_t`.

The original proposal defined two overloads for `operator==` (different
argument order) as well as `operator!=`. This has been removed by
[P1614](https://wg21.link/p1614).

Differential Revision: https://reviews.llvm.org/D119620