QBitArray: DRY operator<<(QDataStream&)

Drag the common parts out of the if-else. This also means that
qsizetype size() is no longer truncated in the assignment to quint32
len, but that doesn't fix the problem that we create invalid Qt 5
streams when the QBitArray has more than INT_MAX bits in it. That is
for a follow-up patch (which is why the {} stay for now).

As a drive-by, replace verbose constData() with data() (the QByteArray
is already const).

Pick-to: 6.7 6.6 6.5 6.2
Change-Id: Iabf816e16f823bb3959469a635a21339242f36fc
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/corelib/tools/qbitarray.cpp

@@ -898,19 +898,15 @@ QBitArray operator^(const QBitArray &a1, const QBitArray &a2)
 
 QDataStream &operator<<(QDataStream &out, const QBitArray &ba)
 {
+    const qsizetype len = ba.size();
     if (out.version() < QDataStream::Qt_6_0) {
-        quint32 len = ba.size();
-        out << len;
-        if (len > 0)
-            out.writeRawData(ba.d.constData() + 1, ba.d.size() - 1);
-        return out;
+        out << quint32(len);
     } else {
-        quint64 len = ba.size();
-        out << len;
-        if (len > 0)
-            out.writeRawData(ba.d.constData() + 1, ba.d.size() - 1);
-        return out;
+        out << quint64(len);
     }
+    if (len > 0)
+        out.writeRawData(ba.d.data() + 1, ba.d.size() - 1);
+    return out;
 }
 
 /*!