QCollatorSortKey: use QExplicitlySharedDataPointer as pImpl

The key is immutable except for assignment and swap,
so don't run the risk of auto-detaching and use the
explicit-detach version of QSharedDataPointer.

Change-Id: Ib2cfe5981e6dfe375d6208289ff58247ef9d4870
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/tools/qcollator.h

@@ -69,7 +69,7 @@ public:
 protected:
     QCollatorSortKey(QCollatorSortKeyPrivate*);
 
-    QSharedDataPointer<QCollatorSortKeyPrivate> d;
+    QExplicitlySharedDataPointer<QCollatorSortKeyPrivate> d;
 
 private:
     QCollatorSortKey();