Add a build/test workflow for Windows

We've seen several downstream build breaks from torch-mlir due to missing test coverage. Having at least a nightly CI build using GitHub-hosted runners would provide earlier signal for build issues.

## Existing workflows

* https://github.com/llvm/torch-mlir/blob/main/.github/workflows/ci.yml only runs on Linux
* https://github.com/llvm/torch-mlir/blob/main/.github/workflows/buildRelease.yml runs on multiple platforms but was last triggered 1 year ago (https://github.com/llvm/torch-mlir/actions/workflows/buildRelease.yml)

## Expanding to new platforms

I see scripts used by ci.yml that could be forked or generalized:

* https://github.com/llvm/torch-mlir/blob/main/build_tools/ci/build_posix.sh
* https://github.com/llvm/torch-mlir/blob/main/build_tools/ci/test_posix.sh

A workflow could also be added that runs commands directly. I've been moving https://github.com/iree-org/iree (a downstream user of torch-mlir) away from such scripts, instead opting to make the build system work better out of the box using default options, with things like ccache and the choice of compiler (e.g. gcc/clang) delegated to the user's choice or a github action that configures environment variables.

Start by reading .github/workflows/ci.yml and buildRelease.yml, then compare build_tools/ci/build_posix.sh and test_posix.sh with the project's Windows build needs. Add a GitHub-hosted Windows workflow with at least nightly build and test coverage, and verify that it provides earlier signal for Windows build issues.

Written by the indexing model from the issue text.