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

[via llvm-commits]

cmtice wrote:

> Can we also pass `-Wno-unsafe-buffer-usage` and `-Wno-old-style-cast`? I'm seeing about 10k instances of each in the logs.
> 

Done.

> Can you also file a Github issue to track the warning cleanup?

Done: https://github.com/llvm/llvm-project/issues/165045

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