Revert "Bump up to LLVM/Clang 7."
Revert "Bump up to LLVM/Clang 7."

This reverts commit ae5e28891dfde22e13db2b402a5cf4303aaefa3a.

nth10sd/laniakea,MozillaSecurity/laniakea,nth10sd/laniakea,MozillaSecurity/laniakea