[llvm] [CI] Update Windows premerge testing to use clang-cl.exe (PR #164900)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

Other than that, LGTM. <60 minutes total for a cold build is pretty sweet.

https://github.com/llvm/llvm-project/pull/164900