Clang: Don't index already indexed symbols

We don't index system headers any more and introduce a first step to
decrease double indexing. For that we introduces the SourcesManager which
so far tells you only if a file was already indexed for a certain time
stamp.

Change-Id: Icde54465693ca84a622764c595635cac365c0111
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/tools/clangrefactoringbackend/source/collectmacrossourcefilecallbacks.cpp

@@ -42,7 +42,8 @@ bool CollectMacrosSourceFileCallbacks::handleBeginSource(clang::CompilerInstance
                 m_sourceDependencies,
                 m_filePathCache,
                 compilerInstance.getSourceManager(),
-                compilerInstance.getPreprocessorPtr());
+                compilerInstance.getPreprocessorPtr(),
+                m_sourcesManager);
 
     compilerInstance.getPreprocessorPtr()->addPPCallbacks(std::move(callbacks));