Data flow: Synthesize post-update nodes for callback arguments inside summarized callables

[hvitved]

If we have a flow summary for, say, a method that passes its first argument into the first parameter of its second argument:

input = Argument[0] and
output = Parameter[0].Argument[1]

and we are modeling the call-back as being passed into itself (as a self/this argument), then we are currently missing a post-update node for the implicit self argument. Adding this is relevant when modeling captured variable flow using field flow.

[michaelnebel]

C# Looks plausible to me.

[aschackmull]

LGTM! I also tested locally with a small qltest. I'll put that up in a PR when this is merged.