If you are working from a git clone of the “monorepo” then you should be able to do a git-diff from the top level directory (parent of the llvm and clang directories) and post that as a single patch.
If you are working from an SVN checkout, it’s probably better to generate two diffs and post them separately. I’d post the LLVM part first, and then when you post the Clang part, point to the LLVM part.