Comment 5 for bug 1792873

Duh, I blame the source naming change (previously it was llvm-toolchain-6.0, now it's just -7 instead of -7.0, ugh). Anyway, nevermind me!

I think I'm fine with the changes introduced and the testing so far. It's always better to have a more up-to-date stack. I also confirmed with Adam that we're ok with it. So: approving the FFe.