[XQuery] IBM-XQ-024: Computed PI constructors

(IBM-XQ-024) Section 3.7.3.5, Computed PI Constructors: The name 
expression of a Computed PI constructor should undergo atomization, and 
should accept values of type xdt:untypedAtomic, in order to be consistent 
with the name expression of a Computed Attribute Constructor.

--Don Chamberlin