[asgerf]

Implements data flow support for Swift's short-hand returns.

When the body of a function consists of a single expression that resolves to a value, that value is implicitly the return value of the function. For example,

func foo() -> String { "42" }

is the same as

func foo() -> String { return "42" }

This is currently implemented on the QL side as the desugaring rule can't be implemented nicely with Yeast, but we might want to desugar it in the future, when Yeast offers better support for subtype checks.