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/symbolscollector.h

@@ -28,6 +28,7 @@
 #include "clangtool.h"
 #include "collectmacrossourcefilecallbacks.h"
 #include "collectsymbolsaction.h"
+#include "sourcesmanager.h"
 #include "symbolscollectorinterface.h"
 
 #include <filepathcachingfwd.h>
@@ -62,6 +63,7 @@ private:
     std::shared_ptr<IndexDataConsumer> m_indexDataConsumer;
     CollectSymbolsAction m_collectSymbolsAction;
     CollectMacrosSourceFileCallbacks m_collectMacrosSourceFileCallbacks;
+    SourcesManager m_sourcesManager;
     FilePathCachingInterface &m_filePathCache;
 };