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

[Alexis Engelke via llvm-commits]

aengelke wrote:

@Abhishek-Varma Looks like a downstream problem in [iree_llvm](https://github.com/iree-org/iree/blob/main/build_tools/cmake/iree_llvm.cmake). It's not possible to just set some CMake values and expect things to work; you also need to run the code in HandleLLVMOptions.cmake. I don't think we support direct access to functions in AddLLVM (and transitively though AddMLIR) without running the outer skeleton.

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