Remove all class-level [[nodiscard]] from the code-base

A class-level [[nodiscard]] used to be the only way to get a waring
for code such as

   QMutexLocker(&mutex);

with original C++17 means. This was because a few of our compilers
would warn about the presence of [[nodiscard]] on ctors, which is
really the semantics we want: we don't want to prevent users from
passing QMutexLocker out of functions and users of those functions
from ignoring the return value, if they so choose. That should be the
choice of the author of the function returning such types, not ours.

So QUIP-0019 makes class-level [[nodiscard]] conditional on proper
rationale in the user docs (or the commit message in case of private
API). Since none of the existing uses really strikes this author as
particularly convincing, remove them all.

All these classes have gotten Q_NODISCARD_CTOR on all their ctors, so
we continue to provide the same true positive warnings, minus the
false positives when returning from functions, at least on the
majority of compilers (and it's not as if all compilers interpreted a
class-level [[nodiscard]] as a trigger to warn on the initial example
of this commit message).

Task-number: QTBUG-104164
Pick-to: 6.6
Change-Id: I163356486e7c80f9d69bf67023010a88233fe662
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qfile.h

@@ -37,7 +37,7 @@ Q_CORE_EXPORT bool qEnableNtfsPermissionChecks() noexcept;
 Q_CORE_EXPORT bool qDisableNtfsPermissionChecks() noexcept;
 Q_CORE_EXPORT bool qAreNtfsPermissionChecksEnabled() noexcept;
 
-class [[nodiscard]] QNtfsPermissionCheckGuard
+class QNtfsPermissionCheckGuard
 {
     Q_DISABLE_COPY_MOVE(QNtfsPermissionCheckGuard)
 public:

src/corelib/kernel/qobject.h

@@ -439,7 +439,7 @@ inline QBindingStorage *qGetBindingStorage(QObject *o)
 Q_CORE_EXPORT QDebug operator<<(QDebug, const QObject *);
 #endif
 
-class [[nodiscard]] QSignalBlocker
+class QSignalBlocker
 {
 public:
     Q_NODISCARD_CTOR

src/corelib/kernel/qproperty.h

@@ -53,7 +53,7 @@ Q_CORE_EXPORT void beginPropertyUpdateGroup();
 Q_CORE_EXPORT void endPropertyUpdateGroup();
 }
 
-class [[nodiscard]] QScopedPropertyUpdateGroup
+class QScopedPropertyUpdateGroup
 {
     Q_DISABLE_COPY_MOVE(QScopedPropertyUpdateGroup)
 public:
@@ -287,7 +287,7 @@ private:
 };
 
 template <typename Functor>
-class [[nodiscard]] QPropertyChangeHandler : public QPropertyObserver
+class QPropertyChangeHandler : public QPropertyObserver
 {
     Functor m_handler;
 public:
@@ -314,7 +314,7 @@ public:
     }
 };
 
-class [[nodiscard]] QPropertyNotifier : public QPropertyObserver
+class QPropertyNotifier : public QPropertyObserver
 {
     std::function<void()> m_handler;
 public:

src/corelib/kernel/qproperty_p.h

@@ -94,7 +94,7 @@ struct QPropertyBindingDataPointer
     }
 };
 
-struct [[nodiscard]] QPropertyObserverNodeProtector
+struct QPropertyObserverNodeProtector
 {
     Q_DISABLE_COPY_MOVE(QPropertyObserverNodeProtector)
 

src/corelib/thread/qmutex.h

@@ -216,7 +216,7 @@ bool QRecursiveMutex::tryLock(int timeout) QT_MUTEX_LOCK_NOEXCEPT
 #endif
 
 template <typename Mutex>
-class [[nodiscard]] QMutexLocker
+class QMutexLocker
 {
 public:
     Q_NODISCARD_CTOR
@@ -313,7 +313,7 @@ private:
 class QRecursiveMutex : public QMutex {};
 
 template <typename Mutex>
-class [[nodiscard]] QMutexLocker
+class QMutexLocker
 {
 public:
     Q_NODISCARD_CTOR

src/corelib/thread/qorderedmutexlocker_p.h

@@ -28,7 +28,7 @@ QT_BEGIN_NAMESPACE
   Locks 2 mutexes in a defined order, avoiding a recursive lock if
   we're trying to lock the same mutex twice.
 */
-class [[nodiscard]] QOrderedMutexLocker
+class QOrderedMutexLocker
 {
 public:
     Q_NODISCARD_CTOR
@@ -120,7 +120,7 @@ private:
 
 #else
 
-class [[nodiscard]] QOrderedMutexLocker
+class QOrderedMutexLocker
 {
 public:
     Q_DISABLE_COPY(QOrderedMutexLocker)

src/corelib/thread/qsemaphore.h

@@ -61,7 +61,7 @@ bool QSemaphore::tryAcquire(int n, int timeout)
 }
 #endif
 
-class [[nodiscard]] QSemaphoreReleaser
+class QSemaphoreReleaser
 {
 public:
     Q_NODISCARD_CTOR

src/corelib/tools/qatomicscopedvaluerollback_p.h

@@ -23,7 +23,7 @@
 QT_BEGIN_NAMESPACE
 
 template <typename T>
-class [[nodiscard]] QAtomicScopedValueRollback
+class QAtomicScopedValueRollback
 {
     std::atomic<T> &m_atomic;
     T m_value;

src/corelib/tools/qscopedvaluerollback.h

@@ -9,7 +9,7 @@
 QT_BEGIN_NAMESPACE
 
 template <typename T>
-class [[nodiscard]] QScopedValueRollback
+class QScopedValueRollback
 {
 public:
     Q_NODISCARD_CTOR

src/corelib/tools/qscopeguard.h

@@ -13,7 +13,7 @@
 QT_BEGIN_NAMESPACE
 
 template <typename F>
-class [[nodiscard]] QScopeGuard
+class QScopeGuard
 {
 public:
     Q_NODISCARD_CTOR