We want to remove llvmPackages_6, but it is the only version GHC 8.6.5
supports. Luckily, we actually don't need LLVM in any case, since all
X86 architectures have native codegen for Darwin and Linux, as well as
POWER for Linux. Consequently, we can just pass `null` and add an extra
assert to make this more transparent to future tinkerers.
