QMutex: remove QMutexPrivate from the public-ish API

Pass the pointer as a void* to avoid having "QMutexPrivate" in the ABI
that user content calls. To have such a private type in ABI used by
inlines in user code required an ELFIGNORE in qmutex_p.h. This change
removes that need for Qt 7.

We can't add an #ifdef for the comment block because "ignore-next" would
then apply to the #endif line. Therefore, a ### Qt7 comment will have to
suffice. It's harmless if we forget to do it.

Change-Id: I83e6ddaf775e71eacb42fffd5cd6b87527b9c08b
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/corelib/compat/removed_api.cpp

@@ -1274,6 +1274,15 @@ QByteArray QMetaEnum::valueToKeys(int value) const
 }
 
 
+#include "qmutex.h"
+
+#if QT_CONFIG(thread)
+void QBasicMutex::destroyInternal(QMutexPrivate *d)
+{
+    return destroyInternal(static_cast<void *>(d));
+}
+#endif
+
 #include "qstring.h"
 
 QString QString::arg(qlonglong a, int fieldWidth, int base, QChar fillChar) const

src/corelib/thread/qmutex.cpp

@@ -101,8 +101,9 @@ static inline QMutexPrivate *dummyFutexValue()
 
     \warning Destroying a locked mutex may result in undefined behavior.
 */
-void QBasicMutex::destroyInternal(QMutexPrivate *d)
+void QBasicMutex::destroyInternal(void *ptr)
 {
+    auto d = static_cast<QMutexPrivate *>(ptr);
     if (!d)
         return;
     if (!futexAvailable()) {

src/corelib/thread/qmutex.h

@@ -88,7 +88,10 @@ private:
     bool lockInternal(int timeout) QT_MUTEX_LOCK_NOEXCEPT;
 #endif
     void unlockInternal() noexcept;
+#if QT_CORE_REMOVED_SINCE(6, 9)
     void destroyInternal(QMutexPrivate *d);
+#endif
+    void destroyInternal(void *d);
 
     QBasicAtomicPointer<QMutexPrivate> d_ptr;
     static inline QMutexPrivate *dummyLocked() {

src/corelib/thread/qmutex_p.h

@@ -35,6 +35,7 @@ struct timespec;
 
 QT_BEGIN_NAMESPACE
 
+// ### Qt7 remove this comment block
 // We manipulate the pointer to this class in inline, atomic code,
 // so syncqt mustn't mark them as private, so ELFVERSION:ignore-next
 class QMutexPrivate