ThreadPoolThreadReleaser: add Q_NODISCARD_CTOR

QUIP: 19
Pick-to: 6.6
Change-Id: I465a2ef8edc103f0655a7732f3aaaf18748854c4
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

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(); }