[libcxx][ranges] Add `default_sentinel` and `default_sentinel_t`.

Refs https://eel.is/c++draft/default.sentinel and https://eel.is/c++draft/iterator.synopsis

Differential Revision: https://reviews.llvm.org/D103487

GitOrigin-RevId: 065cf3f9d70374d0a02f1e15be32eaaa59f01466
5 files changed