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

[Mehdi Amini via llvm-commits]

joker-eph 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.

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