[llvm] workflows/premerge: Generate a ccache artifact for each pull request (PR #124311)

[Tom Stellard via llvm-commits]

tstellar wrote:

> Do we need separate actions for this? I thought it was possible to implement this by attaching the PR (if we're running against a pr and the number is available) to the cache key, and then setting the ccache action to also look for cache keys without a PR (like the most recent run against `main`)?

You can implement this using the cache, but it's limited to 10 GB (for the whole repo), so that's not really enough to cache all the PRs.  By using an artifact instead of the cache, you can ensure that the cache for each PR will persist long enough to actually be reused when the PR is updated.

https://github.com/llvm/llvm-project/pull/124311