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Adding a clang project is mainly motivated by getting our mingw-w64/clang toolchain for Windows set up (#28716). Clang is a separate project and will replace the llvm one (which we use in parallel for now) as we don't want to build the clang part every time, say, the ming-w64 commit gets updated. Moreover, we need a recent enough clang on the host system for building at least the Stylo part in Firefox anyway. The patches for Windows and general set up is more or less following build/build-clang/clang-8-mingw.json on the mozilla-esr68 branch.