Member predicate Function::mayReturnNormally

Holds if this function may return without panicking, exiting the process, or looping forever.

This predicate is an over-approximation: it may hold for functions that can never return normally, but it never fails to hold for functions that can.

Note this is declared here and not in DeclaredFunction so that library models can override this by extending Function rather than having to remember to extend DeclaredFunction.