Clang: Add defines for llvm

Actually we maybe should use llvm-config --cxxflags but before we do
that we should evaluate possible compile breaks.

Change-Id: I534605c6f61217bfe985bef22af7d1dc9ac79637
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/tools/clangrefactoringbackend/clangrefactoringbackend.pro

@@ -11,6 +11,8 @@ QT -= gui
 LIBS += $$LIBTOOLING_LIBS
 INCLUDEPATH += $$LLVM_INCLUDEPATH
 
+QMAKE_CXXFLAGS += $$LLVM_CXXFLAGS
+
 SOURCES += \
     clangrefactoringbackendmain.cpp