On the flip side, code from permissively-licensed projects (like LLVM/clang) can be incorporated into free software with GPL-incompatible licenses. I personally consider that gain to be more than worth the risk of proprietary forks.

Also, the licenses of neither GCC nor LLVM apply to programs compiled by either project. I don't think that's what you meant by the copyleft sentiment, but the wording of that bit is weird and it's worth clarifying just in case.