Replace uses of std::iterator with explicit using

This patch removes all uses of `std::iterator`, which was deprecated in C++17.
While this isn't currently an issue while compiling LLVM, it's useful for those using LLVM as a library.

For some reason there're a few places that were seemingly able to use `std` functions unqualified, which no longer works after this patch. I've updated those places, but I'm not really sure why it worked in the first place.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D67586