[libc++] [ranges] Add namespace __cpo to ranges::{advance,next,prev}.

The reason for those nested namespaces is explained in D115315:

> AIUI, this keeps the CPO's own type from ADL'ing into the `std::ranges`
> namespace; e.g. `foobar(std::ranges::uninitialized_default_construct)`
> should not consider `std::ranges::foobar` a candidate, even if
> `std::ranges::foobar` is not a CPO itself. Also, of course, consistency
> (Chesterton's Fence, the economist's hundred-dollar bill): if it were
> safe to omit the namespace, we'd certainly want to do it everywhere,
> not just here.

This makes these three niebloids more consistent with the other Ranges
niebloids we've already implemented, such as the `ranges::begin` group
and the `ranges::uninitialized_default_construct` group.

FWIW, we still have three different indentation-and-comment styles
among these three groups.

Differential Revision: https://reviews.llvm.org/D116569