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

[Tom Stellard via llvm-commits]

tstellar wrote:

I've done a major rework of this with the following changes:

* Split the build into 3 jobs to avoid timeouts.
* Added a sccache cache the is specific to each PR.  This should help make testing PR updates faster.

I'm still debugging and addressing some of the review comments, but I think this new approach will be better.

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