Our GitHub CI actions use a custom action for installing LLVM. This should be moved to a separate repository that can be released as a standalone GitHub CI action. A new repository was created for this purpose.

Good first issue process:

• read our contributing guidelines
• claim this issue (assign yourself or comment below)
• copy install-llvm action code to this repository and make potentially necessary changes to release it as a standalone GitHub CI action.
• Create two Pull Requests:
  • A pull request for the GitHub CI action referencing this issue in the description: partial implementation of Move llvm-install GitHub CI action to separate repo #526.
  • After merging the above Pull Request, remove the old install-llvm action code from this repository, replace it with the standalone GitHub CI action, and create a Pull Request in this repository referencing this issue in the description: closes Move llvm-install GitHub CI action to separate repo #526

If this is your first PR, welcome 🎉 😄