[llvm] Propagate the Buildkite pre-merge setup to GitHub Action (PR #81077)

[Tom Stellard via llvm-commits]

tstellar wrote:

> > It would be nice if we could combine this into a single job and select the targets to build as necessary to avoid the issue you mentioned on the Discourse thread. Maybe even an adaptation of the current shell script in .ci that selects stuff for Buildkite? This would probably be necessary in a separate job to not impact the release jobs, but that would be up to @tstellar.
> 
> Yeah: we discussed it here: https://discourse.llvm.org/t/rfc-future-of-windows-pre-commit-ci/76840/34
> 
> I'll leave this PR as a reference, and send another one in this direction for consideration.

@joker-eph Do you have a prototype for this? I was thinking about doing something similar for the Windows CI on GitHub Actions

https://github.com/llvm/llvm-project/pull/81077