Pack QXmlStreamReader's Value struct

Now that QTBUG-102465 has fixed QXmlStreamReader's int/qsizetype mismatches (incl. some bugs), the central Value struct is now

{
     qsizetype pos;   // 8 bytes
     qsizetype len;   // 8 bytes
     qint16 prefix;   // 2 bytes
     ushort c;        // 2 bytes
                      // ========
                      // 20 bytes
}

The prefix is only used for names, for which fastScanName() imposes an upper limit of 4096 on the length, in which case a qint16 len would suffice, too.

For character content, which can reasonably be expected to run for more than 2Gi character, OTOH, there's no need for a prefix (TODO: check c).

Finally, when c is used, len is always 1 and prefix is 0 (TODO: check that's correct).

So we should be able to save some space by adding variadic members to Value:

{
     qsizetype pos;                           // 8 bytes - can't be helped
     union {
         struct {
             qsizetype len; // 8 bytes
         } characters;
         struct {
             qint16 len;    // 2 bytes
             qint16 prefix; // 2 bytes
         } name;
        ushort c;           // 2 bytes
    };                                        // 8 bytes
                                              // ========
                                              // 16 bytes
}

which brings us back to where we were in Qt 5 with struct Value {int, int, int, ushort} (14 bytes + 2 padding).

If it's not clear by context which kind of Value we have, we also need to embed a discriminator, which would be easy in 64-bits (use some bits from pos), but may be tricky in 32-bit (where we really only have one bit to spare in pos - the sign bit).