Konstantin Varlamov 43bc1e5600 [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
2022-02-15 16:59:42 -08:00
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