[DM] Names for abstract Schema types

The data model introduces the name xdt:anyAtomicType to refer to the
abstract superclass of all atomic types.

It would also be useful to have names for the other abstract types implicit
in the data model, specifically:

xdt:anyListType
xdt:anyUnionType
xdt:anyComplexType

These would then be available for use in the SequenceType production. For
example, one could define a template rule

<xsl:template match="attribute(*, xdt:anyListType)">

to match all list-valued attributes.

Michael Kay