[flang] Checklist to precede pull requests

I added a checklist to the C++ Style document for things to check before
submitting a pull request or when responding to a request for comments
on a pull request.

Original-commit: flang-compiler/f18@903420268a5aabbab9acdf8788b4344c395ee597
Reviewed-on: https://github.com/flang-compiler/f18/pull/936