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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@aengelke This is spamming MSVC builds with exception override warnings https://lab.llvm.org/buildbot/#/builders/46/builds/28456
```
[3188/18/878/4083]Building CXX object lib\Analysi...eFiles\LLVMAnalysis.dir\MemoryProfileInfo.cpp.obj
cl : Command line warning D9025 : overriding '/EHs' with '/EHs-'
cl : Command line warning D9025 : overriding '/EHc' with '/EHc-'
[3187/18/879/4083]Building CXX object lib\Analysis\CMakeFiles\LLVMAnalysis.dir\MemoryBuiltins.cpp.obj
cl : Command line warning D9025 : overriding '/EHs' with '/EHs-'
cl : Command line warning D9025 : overriding '/EHc' with '/EHc-'
[3186/18/880/4083]Building CXX object lib\Analysis\CMakeFiles\LLVMAnalysis.dir\LoopInfo.cpp.obj
cl : Command line warning D9025 : overriding '/EHs' with '/EHs-'
cl : Command line warning D9025 : overriding '/EHc' with '/EHc-'
[3185/18/881/4083]Building CXX object lib\Analysis\CMakeFiles\LLVMAnalysis.dir\ObjCARCInstKind.cpp.ob
cl : Command line warning D9025 : overriding '/EHs' with '/EHs-'
cl : Command line warning D9025 : overriding '/EHc' with '/EHc-'
[3184/18/882/4083]Building CXX object lib\Analysi...LLVMAnalysis.dir\MemoryDependenceAnalysis.cpp.obj
cl : Command line warning D9025 : overriding '/EHs' with '/EHs-'
cl : Command line warning D9025 : overriding '/EHc' with '/EHc-'
[3183/18/883/4083]Building CXX object lib\Analysi...les\LLVMAnalysis.dir\ObjCARCAliasAnalysis.cpp.obj
cl : Command line warning D9025 : overriding '/EHs' with '/EHs-'
cl : Command line warning D9025 : overriding '/EHc' with '/EHc-'
[3182/18/884/4083]Building CXX object lib\Analysi...keFiles\LLVMAnalysis.dir\MemorySSAUpdater.cpp.obj
cl : Command line warning D9025 : overriding '/EHs' with '/EHs-'
cl : Command line warning D9025 : overriding '/EHc' with '/EHc-'
[3181/18/885/4083]Building CXX object lib\Analysi...Files\LLVMAnalysis.dir\LoopAccessAnalysis.cpp.obj
cl : Command line warning D9025 : overriding '/EHs' with '/EHs-'
cl : Command line warning D9025 : overriding '/EHc' with '/EHc-'
``` 

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