[llvm] [ADT] Fix MSVC build after iterator C++20 fix (PR #173495)

[Vedran Miletić via llvm-commits]

vedranmiletic wrote:

@jtuyls @kuhar FWIW, I am wondering if some `friend` markings would be a less radical option that would satisfy MSVC.

https://github.com/llvm/llvm-project/pull/173495