[llvm] Remove uses of deprecated `std::iterator`

std::iterator has been deprecated in C++17 and some standard library implementations such as MS STL or libc++ emit deperecation messages when using the class.
Since LLVM has now switched to C++17 these will emit warnings on these implementations, or worse, errors in build configurations using -Werror.

This patch fixes these issues by replacing them with LLVMs own llvm::iterator_facade_base which offers a superset of functionality of std::iterator.

Differential Revision: https://reviews.llvm.org/D131320