Switch a comma operator to a IILE

A comma operator results in a discarded-value expression, meaning the
compiler is going to complain the moment we mark QMutexLocker as
nodiscard. Turn the comma into a functionally equivalent IILE.

Change-Id: I33826902c8471016490aac25160b70c609dafd90
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/plugin/qlibrary.cpp

@@ -548,7 +548,7 @@ void QLibraryPrivate::setLoadHints(QLibrary::LoadHints lh)
 QObject *QLibraryPrivate::pluginInstance()
 {
     // first, check if the instance is cached and hasn't been deleted
-    QObject *obj = (QMutexLocker(&mutex), inst.data());
+    QObject *obj = [&](){ QMutexLocker locker(&mutex); return inst.data(); }();
     if (obj)
         return obj;