Clang: Don't reparse if the macros haven't changed

We have to extend that to include paths too, which will be happen in a
follow up patch.

Change-Id: I7f8ac663ae8588e647fc6a6b5d689a629a28ef65
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/tools/clangrefactoringbackend/source/symbolindexer.h

@@ -52,6 +52,8 @@ public:
     void pathsChanged(const FilePathIds &filePathIds) override;
     void updateChangedPath(FilePathId filePath);
 
+    bool compilerMacrosAreDifferent(const V2::ProjectPartContainer &projectPart) const;
+
 private:
     SymbolsCollectorInterface &m_symbolsCollector;
     SymbolStorageInterface &m_symbolStorage;