Introduced a token cache for the C++ editor.

This should speed things up a bit, because before, the line was tokenized at
least 3 times.

src/plugins/cppeditor/cpphoverhandler.cpp

@@ -254,7 +254,7 @@ void CppHoverHandler::updateHelpIdAndTooltip(TextEditor::ITextEditor *editor, in
         }
 
         // Fetch the expression's code
-        ExpressionUnderCursor expressionUnderCursor;
+        ExpressionUnderCursor expressionUnderCursor(m_modelManager->tokenCache(editor));
         const QString expression = expressionUnderCursor(tc);
 
         const QList<LookupItem> types = typeOfExpression(expression, scope);