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

[Tom Stellard via llvm-commits]

tstellar wrote:

> I do have very strong concerns about this: it does not seem acceptable to me to make steps in a direction where we're reduce the previous support and make it seems that there are two classes of support in the project between subprojects.

The current version of this workflow isn't meant to replace buildkite, I see it as a backup plan until either the existing Windows job on buildkite is fixed or we get more powerful Windows runners on GitHub.

I'm still testing to see how many sub-projects we can realistically enable.

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