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

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

I'm somewhat confused by how this code works, including for the existing LLVM_CONFIG_HAS_RTTI flag. If LLVM_REQUIRES_RTTI is used somewhere, does this depend on whether the last target for which llvm_update_compile_flags is called happens to have that set or not?

Should llvm-config only be using the global LLVM_ENABLE_RTTI instead of this second LLVM_CONFIG_HAS_RTTI variable that also uses LLVM_REQUIRES_RTTI? And if so, can we extract determining the corresponding compiler flag into a separate function that gets explicitly called from the llvm-config cmake instead of relying on the side-effect here?

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