[Workflow] Add Release Repo sync script

Adds a bash script that syncs llvm/llvm-project and llvm/llvm-project-release-prs.
This should run on pushes to any of the repositories release branches.

I will follow this up with a change to the github actions to run this
script.

This breaks out the sync script from: https://reviews.llvm.org/D133476
so we can keep them separate.

Reviewed By: kwk

Differential Revision: https://reviews.llvm.org/D142726