[libc++] Deprecate std::iterator and remove it as a base class

C++17 deprecated std::iterator and removed it as a base class for all
iterator adaptors. We implement that change, but we still provide a way
to inherit from std::iterator in the few cases where doing otherwise
would be an ABI break.

Supersedes D101729 and the std::iterator base parts of D103101 and D102657.

Differential Revision: https://reviews.llvm.org/D103171