ci: Add LLVM/Clang 13.0 nightly CI jobs on Windows

Add jobs covering the `clang-cl` front-end with Ninja and NMake.
Add a job covering the `clang++` front-end with Ninja.
24 jobs for !6938 with ci-clang-windows in 11 minutes and 45 seconds (queued for 4 seconds)
latest merge request