Bug 30585: Provide clang 8 for Tor Browser 9

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.