[llvm] [workflows] Port buildkite Windows config to GitHub actions (PR #82093)

[Aaron Ballman via llvm-commits]

AaronBallman wrote:

> Shall we actually setup a proof of concept custom GCP Windows runner? Then we can just invite Google to actually migrate the buildkite runners to GitHub instead. I seem to recall Microsoft ready to provide some machine as well? (@AaronBallman knows maybe?). Having more beefy machines (+ having sizable local persistent sccache as well). may just be what we need here.

I have two IceLake servers that are in the process of being provisioned at Intel (the servers are available, folks are getting to the point of needing to install an OS and configure the machines). As I understand things, Intel can host the servers but we need someone from community to actually set up and maintain the CI bits on them. It's somewhat unclear as to who would be responsible for things like updating the OS, but I figure details like that can be worked out once we have someone from the LLVM community who is willing to be the point of contact for getting CI working on the servers. I'm happy to be involved in the process, but I don't know enough about CI or have enough bandwidth to own the servers myself; is someone willing to be that point of contact?

https://github.com/llvm/llvm-project/pull/82093