clangml.4.7.0 is now in opam, with the bug fixes/features requested by @n47 and some others. All LLVM/Clang versions up to 14.0.x are supported, as well as OCaml 5.0. The official repo is now on github: GitHub - thierry-martinez/clangml which should ease posting issues and pull requests (and should be more convenient than discussions on this thread!).
Support for the upcoming Clang 15 is planned for the next release that should happen soon (the development version already supports Clang 15).