In the case that the field data is not a SvxDateTimeField or SvxDateField
then the old "aFieldInfo.pFieldItem = nullptr" call will mean that
the pFieldItem is leaked for that element.

The only thing needed later is the existance of a date field, and the field's
position, so encode those two facts as a bool and EPosition and
stop abusing aFieldInfo.pFieldItem to serve as a flag

Change-Id: I6a1c46d32406dbd6865cfcf6d23a3863bada6810