[libc++] Define insert_iterator::iter with ranges::iterator_t.

The `insert_iterator::iter` member is defined as `Container::iterator` but
the standard requires `iter` to be defined in terms of `ranges::iterator_t` as
of C++20. So, if in C++20 or later, define the `iter` member as
`ranges::iterator_t`.

Original patch by Joe Loser!

Differential Revision: https://reviews.llvm.org/D108575