QSqlQueryModel: memoize roleNames()

The return value of this function is constant and some users (QML?)
may call this function often, so memoize the result of the function
instead of re-creating the QHash on every call.

Amends 40206a9f6d7635bb19305d1c8d74908808e3529e.

Not picking to Qt 5, because I'm unsure about the state of magic statics
there, and I don't want to take out the Q_GLOBAL_STATIC sledgehammer.

Pick-to: 6.9 6.8 6.5
Change-Id: I8311e93ea16982c82e8312e1e104d95ed062fe6b
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/sql/models/qsqlquerymodel.cpp

@@ -193,9 +193,10 @@ bool QSqlQueryModel::canFetchMore(const QModelIndex &parent) const
 */
 QHash<int, QByteArray> QSqlQueryModel::roleNames() const
 {
-    return QHash<int, QByteArray> {
+    static const QHash<int, QByteArray> names = {
         { Qt::DisplayRole, QByteArrayLiteral("display") }
     };
+    return names;
 }
 
 /*! \internal