Add a cache for the header/source mapping.

Now that we are iterating all the files in the project (instead of stopping at
the first success), we need a cache to make the switching fast enough on big
projects.

Change-Id: Ib7d63c93b3303c534ca53856d96333a822046ced
Reviewed-by: Leandro Melo <leandro.melo@nokia.com>

src/plugins/cpptools/cpptoolsplugin.h

@@ -44,6 +44,7 @@
 #include <QtCore/QFutureInterface>
 #include <QtCore/QPointer>
 #include <QtCore/QFutureWatcher>
+#include <QtCore/QHash>
 
 QT_BEGIN_NAMESPACE
 class QFileInfo;
@@ -97,6 +98,7 @@ private:
     CppModelManager *m_modelManager;
     QSharedPointer<CppFileSettings> m_fileSettings;
     CppToolsSettings *m_settings;
+    mutable QHash<QString, QString> m_headerSourceMapping;
 };
 
 } // namespace Internal