XdmSimpleExpression for Literal values

[rhdunn]

interface XdmSimpleExpression {
    val derivedType: XdmType
    val constantValue: Any?
}

This provides the data model for representing expressions that can evaluate to a single constant value, specifically OrExpr nodes. This allows support for constant code warnings and folding, as well as other static analysis.

This issue implements XdmSimpleExpression for OrExpr nodes that contain a single Literal node as follows:

1. single XdmAtomicValue child:

   derivedType = child.lexicalValue
   constantValue = child.lexicalRepresentation

2. single XdmSimpleExpression child:

   derivedType = child.derivedType
   constantValue = child.constantValue

3. all other cases:

   derivedType = XsUntyped // may not be an atomic type, so cannot be XsUntypedAtomic
   constantValue = null // cannot determine a constant value

Simple Expression Nodes:

• [ ] OrExpr
• [ ] AndExpr
• [ ] ComparisonExpr
• [ ] FTContainsExpr
• [ ] XQueryStringConcatExprImpl
• [ ] XQueryRangeExprImpl
• [ ] XQueryAdditiveExprImpl
• [ ] XQueryMultiplicativeExprImpl
• [ ] XQueryUnionExprImpl
• [ ] XQueryIntersectExceptExprImpl
• [ ] XQueryInstanceofExprImpl
• [ ] XQueryTreatExprImpl
• [ ] XQueryCastableExprImpl
• [ ] XQueryCastExprImpl
• [ ] XQueryArrowExprImpl
• [ ] XQueryUnaryExprImpl
• [ ] XQuerySimpleMapExprImpl
• [ ] XQueryPathExprImpl
• [ ] XQueryRelativePathExprImpl
• [x] XQueryPostfixExprImpl

[rhdunn]

The design and optree model for this has changed. Additionally, simple (value) type forwarding is no longer needed as a result of the PSI tree simplification.