[libc++] Add _ITER_CONCEPT and _ITER_TRAITS implementations from C++20

These traits are currently unused because we don't implement ranges.
However, their addition is part of ongoing work to allow libc++
to optimize on user-provided contiguous iterators.

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Cr-Mirrored-Commit: 6624fcba43be88fd357c74435a4d6462858c8fed
4 files changed