#232 Static type checking of untypedAtomic

[Michael Kay]

A static type error may be reported if an operand of an
expression is known at compile time to have the type
xdt:untypedAtomic, even though the expression would
succeed at runtime.

This will only happen if a variable (or function
parameter) is explicitly declared with the type
xdt:untypedAtomic. As a circumvention, change the type
declaration to xdt:anyAtomicType.

This affects both XSLT and XQuery.

Source code fixed (in TypeChecker). Test cases added:
xqmp153, xqmp154.