Redeclare getPrimitiveItemType() to return a more specific result type
Get the primitive item type corresponding to this item type.
For anyAtomicValue and union types it is Type.ATOMIC_VALUE. For numeric it is Type.NUMBER.
For other atomic types it is the primitive type as defined in XML Schema,
except that integer, xs:dayTimeDuration, and xs:yearMonthDuration
are considered to be primitive types.
the corresponding primitive type (this is an instance of BuiltInAtomicType in all cases
except where this type is xs:error. The class ErrorType does not inherit from BuiltInAtomicType
because of multiple inheritance problems).