Compile clang closer to how Mozilla does it

We compile clang differently to how Mozilla does it: Mozilla has an elaborate setup with three stages (if needed) (see: https://hg.mozilla.org/releases/mozilla-esr60/file/248ca5c585f8/build/build-clang/build-clang.py) while we essentially "just" do what is outlined on https://clang.llvm.org/get_started.html.

We should change that and get our toolchain closer to what Mozilla provides.