[clang] Add a Windows build in the Clang pre-commit CI

This patch adds a CI job for Clang on Windows that is separate from
the monolithic job that gets added automatically via the Phabricator
integration with Buildkite. This way, we will retain the Windows testing
for Clang when we move to GitHub Pull Requests.

Differential Revision: https://reviews.llvm.org/D158995