[libc++][NFC] Simplify `optional::iterator` (#183230)

- Rename `__optional_iterator` into `__optional_iterator_base` and make
it part of the `__optional_{meow}_base` inheritance chain. This allows
us to get rid of a sketchy-looking downcast and shorten some code.

NOKEYCHECK=True
GitOrigin-RevId: a2231642b341c71ba184af2b3831502fded401da