Setup LLVM toolchain

This is a GitHub action to setup LLVM toolchain.

In actions.yml:

steps:
  - uses: numworks/setup-llvm@latest

How to update this action

1. Create a new GitHub release, and name it after the Arm toolchain release (e.g. 2022-08). You will need to create a new git tag, just give it the same name.
2. Attach all tarballs from Arm to this release (serves as a mirror for future GitHub action runs), and note their final URL
3. Update the first three constants in index.js to point to the URL you just noted
4. Remove your local node_modules directory and run npm install
5. Commit index.js and your local node_modules folder, and push to master
6. Update the newly created tag (e.g. 2022-08) as well as the latest tag to point to this commit