Clang: Disable RTTI for refactoring backend

LLVM is normally not compiled with RTTI so we get link errors. We should
use "llvm-config --cxxflags" but before we can use it we should be sure
that we don't break something.

Change-Id: If45afcd625273230960a2c2065877f2512a53209
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/tools/clangrefactoringbackend/clangrefactoringbackend.pro

@@ -11,7 +11,7 @@ QT -= gui
 LIBS += $$LIBTOOLING_LIBS
 INCLUDEPATH += $$LLVM_INCLUDEPATH
 
-QMAKE_CXXFLAGS += $$LLVM_CXXFLAGS
+QMAKE_CXXFLAGS += $$QMAKE_CXXFLAGS_RTTI_OFF $$LLVM_CXXFLAGS
 
 SOURCES += \
     clangrefactoringbackendmain.cpp