Mark QThread::currentThreadId() as a pure function

It always returns the same information for each thread it is called
in. But since it's different per thread, we don't think it's
const. pthread_self() on Linux is marked const, though we think it
really ought to be pure. On other OSes, the annotation isn't present,
but the we can assume function is so.

Change-Id: Ifea6e497f11a461db432ffff1448c2b37d94d5f3
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/thread/qthread.h

@@ -56,7 +56,7 @@ class Q_CORE_EXPORT QThread : public QObject
 {
     Q_OBJECT
 public:
-    static Qt::HANDLE currentThreadId() Q_DECL_NOTHROW;
+    static Qt::HANDLE currentThreadId() Q_DECL_NOTHROW Q_DECL_PURE_FUNCTION;
     static QThread *currentThread();
     static int idealThreadCount() Q_DECL_NOTHROW;
     static void yieldCurrentThread();