Change QVector's in-memory data layout

Change QVector's in-memory data layout

The new layout matches that of QByteArrayData and QStringData, except
for offset which is measured from the beginning of QVectorData, whereas
in those classes it (still?) references the end of the header data.

The new layout uses an extra member for storing an offset into the data,
which will allow introducing QVector::fromRawData, similar to the same
functionality already existing in QString and QByteArray.

By not using an actual array to index array members, we also steer clear
of GCC bug #43247:

    http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43247

Change-Id: I408915aacadf616b4633bbbf5cae1fc19e415087
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>