ThreadPoolThreadReleaser: add Q_NODISCARD_CTOR

QUIP: 19
Change-Id: I465a2ef8edc103f0655a7732f3aaaf18748854c4
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit df96d39c048a467213c6170dc8e225116f31a7e3)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/thread/qfutureinterface.cpp

@@ -30,6 +30,7 @@ namespace {
 class ThreadPoolThreadReleaser {
     QThreadPool *m_pool;
 public:
+    Q_NODISCARD_CTOR
     explicit ThreadPoolThreadReleaser(QThreadPool *pool)
         : m_pool(pool)
     { if (pool) pool->releaseThread(); }