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

[Jakub Kuderski via llvm-commits]

kuhar wrote:

Same, I can see failures downstream even with the commit above, which causes:
 ```
  Found erroneous configuration for source file PassPlugin.cpp

  LLVM's build system enforces that all source files are added to a build
  target, that exactly one build target exists in each directory, and that
  this target lists all files in that directory.  If you want multiple
  targets in the same directory, add PARTIAL_SOURCES_INTENDED to the target
  specification, though it is discouraged.
```

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