Remove switch statements in PreorderAST methods

[kkjeer]

PR #1030 introduces three new Node subclasses to PreorderAST: UnaryOperatorNode, MemberNode, and ImplicitCastNode. These node kinds all have class-specific behavior for PreorderAST methods such as Compare, CompareNodes, Coalesce, Sort, and ConstantFold, and these methods currently use switch statements based on the kind of the Node * argument. These methods should be moved into their respective Node classes so that PreorderAST can simply call N->Sort(), etc. for a Node *N.

[kkjeer]

There are a few issues to consider once this code is cleaned up:

1. Can we refactor the call to AttachZero when the parent is null? This call is made in order to canonicalize an expression e to e + 0. This is needed by bounds widening so that if (*p) widens the bounds of p if the declared bounds of p are (p, p + 0).
2. For integer constant expressions of the form +e or -e, can we create a UnaryOperatorNode, or is it necessary to create a LeafExprNode so that constant folding works as expected?