[libc++] Remove assumptions that std::array::iterator is a raw pointer (#74624)

This patch removes assumptions that std::array's iterators are raw
pointers in the source code and in our test suite. While this is true
right now, this doesn't have to be true and ion the future we might want
to enable bounded iterators in std::array, which would require this
change.

This is a pre-requisite for landing #74482