[libc++] Simplify __unwrap_iter a bit (#175153)

`__unwrap_iter` doesn't need to SFINAE away, so we can just check inside
the function body whether an iterator is copy constructible. This
reduces the overload set, improving compile times a bit.

GitOrigin-RevId: 7581c7018bc47f07b0acc5e9fa7f24d2085ea016