Debian 11's libstdc++ (GCC 10) lacks <format> even with clang-18.
Arch ships clang-18 and the latest libstdc++ out of the box.
Container-based runner with pacman for dead-simple dependency install.
<llvm.sh 18 all> installs clang-18 with full C++23 support.
sort -V ensures the highest clang version is selected when
multiple are installed. Removes -stdlib=libc++ (clang-18
provides <format> via its own header, no flag needed).
- Tries apt clang-18/17/16/15 first (Ubuntu 22.04+)
- Falls back to LLVM repo install for Ubuntu 20.04
- Auto-detects installed clang version for CMake compiler flags
- Both CMake 3.28 (binary download) and C++23 compiler required