CppEditor: Avoid a FilePath <-> QString roundtrip

Change-Id: Ia7860e04090aee8b7fae18cdc5f2d85880f29d69
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/cppeditor/cppeditorwidget.cpp

@@ -1165,9 +1165,7 @@ bool CppEditorWidget::isOldStyleSignalOrSlot() const
     const QString content = textDocument()->plainText();
 
     return CppEditor::CppModelManager::instance()
-               ->getSignalSlotType(textDocument()->filePath().toString(),
-                                        content.toUtf8(),
-                                        tc.position())
+               ->getSignalSlotType(textDocument()->filePath(), content.toUtf8(), tc.position())
            == CppEditor::SignalSlotType::OldStyleSignal;
 }