[libc++] Fix `_IterOps::__iter_move` to support proxy iterators.

The return type was specified incorrectly for proxy iterators that
define `reference` to be a class that implicitly converts to
`value_type`. `__iter_move` would end up returning an object of type
`reference` which would then implicitly convert to `value_type`; thus,
the function will return a `value_type&&` rvalue reference to the local
temporary.

Differential Revision: https://reviews.llvm.org/D130197