Isn't RMS' main issue with LLVM that people can create proprietary plugins and those plugins benefit when LLVM benefits? How exactly is this different than if a company were to make proprietary extensions to Emacs and sell them? Does the Emacs license forbid against non-free extensions? Ultimately, it's not LLVM's problem if people create non-free extensions. You could argue all of Gnu/Linux is bad because some proprietary software runs on it. I don't think that's a route we want to take.