Revert "QFlags: befriend the QDataStream serialization operators"

This reverts commits 729c0960b84183bcf9576f974cb5e3b3c3fdc6ef and
348b680ae56e92f061424a10c4baa726cf7f15d2. Friendship is not required.

The removal of the forward-declaration in qflags.h implied adding it
where it had been required but not present.

Change-Id: Ie17963f95bb5fa98fd75fffd7e6e5289b58f1960
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
(cherry picked from commit c7a069870ae80f47b32479dc1f8f7161a3238201)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/global/qflags.h

@@ -12,7 +12,6 @@
 
 QT_BEGIN_NAMESPACE
 
-class QDataStream;
 template<typename Enum> class QFlags;
 
 class QFlag
@@ -206,10 +205,6 @@ private:
         return (it == end ? Int(0) : (Int(*it) | initializer_list_helper(it + 1, end)));
     }
 
-#ifndef QT_NO_DATASTREAM
-    template <typename E> friend QDataStream &operator<<(QDataStream &, QFlags<E>);
-    template <typename E> friend QDataStream &operator>>(QDataStream &, QFlags<E> &);
-#endif
     using Base::i;
 };
 

src/corelib/serialization/qdatastream.h

@@ -517,11 +517,16 @@ inline QDataStream &QDataStream::operator<<(quint64 i)
 
 template <typename Enum>
 inline QDataStream &operator<<(QDataStream &s, QFlags<Enum> e)
-{ return s << e.i; }
+{ return s << e.toInt(); }
 
 template <typename Enum>
 inline QDataStream &operator>>(QDataStream &s, QFlags<Enum> &e)
-{ return s >> e.i; }
+{
+    typename QFlags<Enum>::Int i;
+    s >> i;
+    e = QFlags<Enum>::fromInt(i);
+    return s;
+}
 
 template <typename T>
 typename std::enable_if_t<std::is_enum<T>::value, QDataStream &>

src/corelib/text/qchar.h

@@ -11,7 +11,7 @@
 
 QT_BEGIN_NAMESPACE
 
-
+class QDataStream;
 class QString;
 
 struct QLatin1Char

src/corelib/tools/qline.h

@@ -8,6 +8,7 @@
 
 QT_BEGIN_NAMESPACE
 
+class QDataStream;
 class QLineF;
 
 /*******************************************************************************

src/corelib/tools/qmargins.h

@@ -14,6 +14,7 @@ QT_BEGIN_NAMESPACE
 
 QT_ENABLE_P0846_SEMANTICS_FOR(get)
 
+class QDataStream;
 class QMarginsF;
 
 /*****************************************************************************

src/corelib/tools/qpoint.h

@@ -19,6 +19,7 @@ QT_BEGIN_NAMESPACE
 
 QT_ENABLE_P0846_SEMANTICS_FOR(get)
 
+class QDataStream;
 class QPointF;
 
 class QPoint