Swift: Model property getters, setters and observers as callables

[MathiasVP]

(Part of https://github.com/github/codeql-c-team/issues/903.)

This is the first of two dataflow PRs for field flow for Swift. It models property getters, setters, and observers as callables in the dataflow library. At the moment, this doesn't give us any field flow, but once we add read and store steps the dataflow library will see these getters and setters as read and writes steps.

Note that there's a slight error in the way we model the arguments to a didSet observer: we're passing the new value as an argument to the didSet observer, but the didSet observer actually takes the old value as an argument. I plan on fixing this in a later PR.

Finally, this PR also fixes an issue with multiple toStrings. The toString predicate had multiple results because NormalParameterNode was extending SsaDefinitionNode instead of SsaDefinitionNodeImpl.