Debugger: Fix marking values as changed

Correctly update the cache before handling new information
to be able to display different colors for values to mark
them as unchanged or changed.

Fixes: QTCREATORBUG-29344
Change-Id: I1af9da4947d544248e5217f4e53f6f4f1c598bda
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/qml/qmlengine.cpp

@@ -1228,6 +1228,7 @@ void QmlEnginePrivate::handleEvaluateExpression(const QVariantMap &response,
     QVariant bodyVal = response.value(BODY).toMap();
     QmlV8ObjectData body = extractData(bodyVal);
     WatchHandler *watchHandler = engine->watchHandler();
+    watchHandler->resetValueCache();
 
     auto item = new WatchItem;
     item->iname = iname;