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

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

I also hit a linking error on my downstream project. I have to add `-fno-rtti` manually.


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