ci: Upgrade to LLVM Clang 16 to support C++20 features on macOS 13 and 14; Ensure LLVM Toolchain Consistency; Add macOS 15 to GitHub workflow.

ci: Upgrade to LLVM Clang 16 to support C++20 features on macOS 13 and 14; Ensure LLVM Toolchain Consistency; Add macOS 15 to GitHub workflow. #2389