[llvm][utils] Add a script to use PRs over pushing to main (#166473)

As discussed on discourse

https://discourse.llvm.org/t/rfc-require-pull-requests-for-all-llvm-project-commits/88164

This is an attempt at a script to automatically make and land PRs.

It creates a branch (or a series of branches for stacks), and lands them
one by one, without waiting for review or checks to pass.

It supports --auto-merge, for single PRs. For stacks, use Graphite or
some other tool that supports auto-merging stacks robustly.

It can work with GitHub tokens for https or ssh keys.

Example:
```console
GITHUB_TOKEN=$(gh auth token) python3 git-llvm-push.py --upstream-remote origin
```

Author: ilovepi