[llvm] [Workflow] make code-format-helper.py mypy-safe (NFC) (PR #69691)

[Ryan Prichard via llvm-commits]

rprichard wrote:

I noticed that I forgot an annotation on `DarkerFormatHelper.instructions`. I added another commit to fix that. This file now passes `mypy --strict code-format-helper.py`, which does complain about functions that lack a return type annotation.


https://github.com/llvm/llvm-project/pull/69691