[llvm] Workflows: Add composite actions for managing sccache caches for a PR (PR #101578)

[Tom Stellard via llvm-commits]

tstellar wrote:

> Will the cache always be empty the first time we build a PR? Shouldn't we want a cache from whatever basenline as a fallback so that we can get some cache hits when we build the PR the first time?

In the first run, this action will do nothing, so whatever cache you had in place when you ran the ccache-action step will still be there.

Here is an example usage: https://github.com/llvm/llvm-project/blob/2ab44973df072f3b5af0ec07feb8a436c3b2ec8e/.github/workflows/ci-tests.yml#L135

In this example we have the ccache-action configured to use a cache shared by other PRs (and potentially pushes to main if we wanted to set it up that way).

https://github.com/llvm/llvm-project/pull/101578