Stop QDateTimePrivate from inheriting QSharedData

We're going to have to stop using QSharedDataPointer because of the
upcoming short-datetime-optimization. By dropping the inheritance, I am
also able to rearrange the members and remove the padding hole that
existed.

The second padding hole was removed in the previous commit, which merged
two 32-bit fields into one.

On 64-bit systems, there's no way around a remaining 4-byte hole
somewhere due to the odd number of 32-bit members. I chose to leave it
in the middle of the struct, before the m_timeZone member, instead of
tail padding, so that the layout of the first members structure are the
same in bootstrapped and non-bootstrapped builds. That should help
debugging the bootstrapped tools in Qt Creator. It's also now the same
in 32- and in 64-bit systems.

Change-Id: Id69569111e7d4e619e22ffff144b1fba708885f7
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/tools/qdatetime.cpp

@@ -2488,7 +2488,8 @@ QDateTimePrivate::QDateTimePrivate(const QDate &toDate, const QTime &toTime, Qt:
                                    int offsetSeconds)
     : m_msecs(0),
       m_status(0),
-      m_offsetFromUtc(0)
+      m_offsetFromUtc(0),
+      ref(0)
 {
     setTimeSpec(toSpec, offsetSeconds);
     setDateTime(toDate, toTime);
@@ -2499,6 +2500,7 @@ QDateTimePrivate::QDateTimePrivate(const QDate &toDate, const QTime &toTime,
                                    const QTimeZone &toTimeZone)
     : m_status(0),
       m_offsetFromUtc(0),
+      ref(0),
       m_timeZone(toTimeZone)
 {
     setSpec(Qt::TimeZone);

src/corelib/tools/qdatetime_p.h

@@ -61,7 +61,7 @@
 
 QT_BEGIN_NAMESPACE
 
-class QDateTimePrivate : public QSharedData
+class QDateTimePrivate
 {
 public:
     // Never change or delete this enum, it is required for backwards compatible
@@ -105,7 +105,8 @@ public:
 
     QDateTimePrivate() : m_msecs(0),
                          m_status(StatusFlag(Qt::LocalTime << TimeSpecShift)),
-                         m_offsetFromUtc(0)
+                         m_offsetFromUtc(0),
+                         ref(0)
     {
     }
 
@@ -116,12 +117,10 @@ public:
     QDateTimePrivate(const QDate &toDate, const QTime &toTime, const QTimeZone & timeZone);
 #endif // QT_BOOTSTRAPPED
 
-    // ### XXX: when the tooling situation improves, look at fixing the padding.
-    // 4 bytes padding
-
     qint64 m_msecs;
     StatusFlags m_status;
     int m_offsetFromUtc;
+    mutable QAtomicInt ref;
 #ifndef QT_BOOTSTRAPPED
     QTimeZone m_timeZone;
 #endif // QT_BOOTSTRAPPED