Add LLVM 16 support and arm64 automated builds

[alinalihassan]

Unfortunately, since GitHub-hosted runners don't support arm64 M1 macOS, we are stuck to using 15.0.4, since LLVM stopped released binaries for Intel-based MacOS. The issue tracking it on GitHub's side is https://github.com/github/roadmap/issues/528.

It's scheduled to release somewhere late 2023.