Three NFC patches in one. The first two are completely mechanical; the third simply shuffles around some declarations so as to shrink the diff when C++20 move_iterator is finally implemented.
[libc++] Rename __i to __current_ in move_iterator. NFC. [libc++] s/_LIBCPP_INLINE_VISIBILITY/_LIBCPP_HIDE_FROM_ABI/g in move_iterator.h. NFC. [libc++] Further small cleanups of move_iterator.h. NFC.