Fix debug build when FEATURE_qml_debug is OFF

This fixes a number of compiler errors, such us "only virtual
member functions can be marked 'final'"

Pick-to: 6.6 6.5
Change-Id: I0c40d0801e48b5f120e992651dea2219efb8a2dc
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
(cherry picked from commit 65ffcb9550)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/qml/qqmllanguage/tst_qqmllanguage.cpp

@@ -6829,6 +6829,7 @@ void tst_qqmllanguage::bareInlineComponent()
     QVERIFY(tab1Found);
 }
 
+#if QT_CONFIG(qml_debug)
 struct DummyDebugger : public QV4::Debugging::Debugger
 {
     bool pauseAtNextOpportunity() const final { return false; }
@@ -6837,6 +6838,9 @@ struct DummyDebugger : public QV4::Debugging::Debugger
     void leavingFunction(const QV4::ReturnedValue &) final { }
     void aboutToThrow() final { }
 };
+#else
+using DummyDebugger = QV4::Debugging::Debugger; // it's already dummy
+#endif
 
 void tst_qqmllanguage::hangOnWarning()
 {