[Format] Work around current vim bugs in clang-format.py

Summary:
Do line/col to byte conversions on the python side rather than relying on vim.
Its calculations are off when text annotations are present:
- https://github.com/vim/vim/issues/5930
- https://github.com/vim/vim/issues/3718 (fixed, but vim 8.1 is still common)

Reviewers: hokein

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78198