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.
Security updates will be applied on September 11th between 0500 PM and 0700 PM, EDT (UTC-0400). This site will have intermittent downtime during that time.
Add jobs covering the clang-cl front-end with Ninja and NMake.
Add a job covering the clang++ front-end with Ninja.