[Github] Add a windows CI container (#118206)
This patch adds a windows CI container mostly based off of the existing
container used for Buildkite
(https://github.com/google/llvm-premerge-checks/blob/a687e33c37fbdcf67b52805c8cf3a8ed145e3243/containers/buildkite-windows/Dockerfile#L1).
This is intended to be a starting point as we transition to Github
Actions with the eventual plan being to build a custom windows toolchain
similar to what we do on Linux.
