[llvm] [Github] Add initial workflow to prune unused user branches (PR #175693)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> Have you experimented with using the associatedPullRequests connection on refs? I tried the following:

I did not try that. My general philosophy with the Github API is to try and keep queries simple (because we've observed plenty of bugs in how the Github API will return/or not return data), and also to do few queries (given we already run into API limits somewhat regularly). If we're looking at refs, it would require about 4x the number of API calls to do a full sweep currently and 1-2x after we do the initial cleanup. I don't think there is too much complexity in manually filtering with branches from the git repository.

The current approach also considers `revert-` branches.

https://github.com/llvm/llvm-project/pull/175693