Comment # 1 on bug 1139584 from Aaron Puchert

You're right, something's amiss. We aren't using the documented flag
LLVM_ENABLE_RTTI (https://llvm.org/docs/CMake.html), but the internal
LLVM_REQUIRES_RTTI. I'll see if we can use the official flag instead.

This probably also applies to llvm6 and llvm7.