[llvm] [CMake] Re-add -fno-rtti to llvm-config --cxxflags (PR #174084)

[Diego Caballero via llvm-commits]

dcaballe wrote:

I'm also hitting some issues with this commit. It looks like a tablegen component in my project that were built with RTTI disabled by default is now built with RTTI enabled so probably some cmake var is not cached or propagated as expected?

https://github.com/llvm/llvm-project/pull/174084