Move `llvm-install` GitHub CI action to separate repo

[Wodann]

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:

• [x] read our contributing guidelines
• [x] 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 #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 #526

If this is your first PR, welcome 🎉 😄

[Larsouille25]

I will try to do it :)

[Larsouille25]

I'm starting to really use GitHub for my projects but i'm not very experienced. I've just create a repo, because I wasn't able to fork this repo. I've even tryed to open a PR but github says me (again) the repository is empty. And I'm not able to push commits to https://github.com/mun-lang/llvm-install-action. So I don't know what to do ?

[shubham2o]

Can you elaborate more about the issue you are facing?

[Larsouille25]

My issue is I want to contribut to this issue (#526), so I wanted to fork the llvm-installation-action. But I can't because there is no files in this repo. So I can't fork, and do pull request.

[Wodann]

Sorry about that. I didn't realise that there'd be such a requirement. I'll submit license files once I get home

[Larsouille25]

Thanks 👍

[Wodann]

I've submitted the files