Div. private RAII/smart ptr classes: mark ctors [[nodiscard]]

The following private APIs are either RAII or smart pointer classes:

- QAutoPointer
- QBoolBlocker
- QFdContainer

QUIP-0019 says to mark RAII and smart pointer class ctors
[[nodiscard]], so do that.

Task-number: QTBUG-104164
Change-Id: Ibc77e6603fadf18ea28428a49635f46a5680b777
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 25e20d5537b2a7682c34a9837076530ad220b3b5)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qobject_p.h

@@ -434,7 +434,9 @@ class QBoolBlocker
 {
     Q_DISABLE_COPY_MOVE(QBoolBlocker)
 public:
-    explicit inline QBoolBlocker(bool &b, bool value = true) : block(b), reset(b) { block = value; }
+    Q_NODISCARD_CTOR explicit QBoolBlocker(bool &b, bool value = true)
+        : block(b), reset(b)
+    { block = value; }
     inline ~QBoolBlocker() { block = reset; }
 
 private:

src/platformsupport/input/evdevkeyboard/qevdevkeyboardhandler_p.h

@@ -101,7 +101,7 @@ class QFdContainer
     int m_fd;
     Q_DISABLE_COPY_MOVE(QFdContainer);
 public:
-    explicit QFdContainer(int fd = -1) noexcept : m_fd(fd) {}
+    Q_NODISCARD_CTOR explicit QFdContainer(int fd = -1) noexcept : m_fd(fd) {}
     ~QFdContainer() { reset(); }
 
     int get() const noexcept { return m_fd; }

src/widgets/dialogs/qdialog_p.h

@@ -103,7 +103,7 @@ template <typename T>
 class QAutoPointer {
     QPointer<T> o;
 public:
-    explicit QAutoPointer(T *t) noexcept : o(t) {}
+    Q_NODISCARD_CTOR explicit QAutoPointer(T *t) noexcept : o(t) {}
     ~QAutoPointer() { delete o; }
 
     T *operator->() const noexcept { return get(); }