[libc++] Mark __wrap_iter's private constructors as explicit.

This is slightly more user-visible than D119894, because the user is
expected to touch `__wrap_iter` directly. But the affected ctors are
non-public, so the user was never expected to be actually calling them.
And I didn't intentionally omit this from D119894; I just didn't
think of it.

Differential Revision: https://reviews.llvm.org/D120937