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

[Alexis Engelke via llvm-commits]

aengelke wrote:

@ronlieb that's unexpected, that should've been there all the time? offload/CMakeLists.txt makes its decision purely on LLVM_ENABLE_RTTI, which was unchanged?

@kuhar but... PassPlugin.cpp is referenced in llvm/lib/Plugins/CMakeLists.txt?

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