Swift: Add some summary queries.

[geoffw0]

I wrote these queries to assess the progress we're making identifying flow sources and sensitive expressions in real world Swift snapshots. I intend to track these things fairly often in the near-mid future, so it would be convenient to have the queries in the public repo. I don't think there's any harm in letting curious users see them as well.

It might be nice to have LinesOfCode.ql and LinesOfUserCode.ql summary queries as well as all (?) other languages do, but I don't think we extract the data for them yet.

[jketema]

Mostly LGTM. Just one description I do not fully understand.

[geoffw0]

In addition to your suggested change I've restricted the count of "Expressions" to ones with locations in code now. It turns out there's quite a lot of locationless stuff that was exaggerating the size of smaller projects (and in turn making me wonder why we weren't finding many results, taint sources etc in them).

[atorralba]

This LGTM. I guess summary queries don't need a docs review.

[geoffw0]

I guess summary queries don't need a docs review.

As far as I can tell summary queries don't usually get full .qhelp documentation, just an explanatory @description. They're not really as visible as our main 'problem' queries so this seems appropriate to me. Happy to add full docs, or get the @descriptions reviewed by the doc team if you would prefer.

[atorralba]

Happy to add full docs, or get the @descriptions reviewed by the doc team if you would prefer.

No, let's keep it consistent with the rest of summary queries. Looks good as it is.