Add support for flow through content of global variables

[smowton]

Note flow via something like sideEffect(&x) is still broken. I think this is because we're lacking a store-step for *x = source(), which should result in x having taint with access path [PointerContent]. This might also involve address operations being able to read taint (i.e. to note that if &x is tainted with [PointerContent] then x is tainted without an access path), but I'm not sure -- in any event I don't address the matter here. I'd suggest tackling that issue by cribbing off C/C++, which has surely addressed (no pun intended) this exact issue.

[owen-mc]

Looks good. I think this needs a QA run. Also, please add failing tests for the cases you mentioned in the PR description, with a brief comment explaining why they fail.

[smowton]

The failing case already exists-- it's the one that still has the MISSING: tag where the others are removed.

[smowton]

(Will DCA then QA)

[smowton]

DCA: one new result (TP, stack trace exposure), no performance effects. Proceeding to QA.

[smowton]

There are too many new results to sensibly check them all, but I checked a sample result from each of the 5 repositories with the most new results, and found they were all doing as intended. Some appeared to be TPs, and some FPs for unrelated reasons, e.g. taint-tracking losing sight of the fact that only one field of a struct carried tainted data, that don't reflect on the change made here.

Performance results were unremarkable. Therefore I recommend merging this.

[smowton]

@owen-mc I've rebased to re-test and added test expectation changes and a change note. Please review and merge if you're happy.

[owen-mc]

I've made an issue to follow up the missing pointer content store step.