Data flow: Introduce 'with/without content' summary components

[hvitved]

This PR introduces

SummaryComponent withoutContent(ContentSet c);
SummaryComponent withContent(ContentSet c);

to FlowSummaryImpl.qll for C#, Java, and Ruby.

The new SummaryComponents are only meant to be used in input specifications, and they gives rise to new synthesized nodes n at which clearsContent(n, c) resp. expectsContent(n, c) holds. This means that we can interpret SummarizedCallable::clearsContent(ParameterPosition ppos, ContentSet cs) as a summary with input stack

SummaryComponentStack::push(SummaryComponent::withoutContent(cs), SummaryComponentStack::argument(ppos))

and output stack

SummaryComponentStack::argument(ppos)

Ultimately, we should get rid of SummarizedCallable::clearsContent(ParameterPosition ppos, ContentSet cs) and use summaries like the above instead, but that will be done follow-up.