C++: Clean up SSA and stop relying on memory edges for iterator flow

[MathiasVP]

This PR ended up being quite a bit larger than expected. I'm sorry!

This PR removes the last dependency on the IR-produced memory edges for dataflow. Some background information:

On main we implement dataflow through iterators by defining SSA reads from (and writes to) iterators by traversing back through the SSA graph to find the preceding call to container.begin(), and we then specify that a write to an iterator is a write to that container (and similar for reads). That is, in order to have flow in an example such as:

auto it = v.begin();
...
*it = source();
...
sink(v.at(0));

we have an SSA write to v at *it = source().

However, in order to conclude that it was obtained from v we need some kind of SSA to find the preceding call to container.begin(). On main we use the SSA information from the IR. However, as we've learned many times, the sound SSA information produced by the IR isn't well-suited for dataflow. In particular:

• It required really specialized dataflow code since that's the only place where we relied on IR-based SSA
• Since it's a sound SSA it means it fails to find the begin call on most real-world code since everything escapes (and thus can't be soundly tracked) according to the current IR alias analysis.
• It became even more ill-suited once we merged sound IR.

This PR fixes this by instead computing iterator flow after dataflow SSA has finished. Thus, we have the full power of the dataflow SSA to find the call to begin. This is then much easier to plug into dataflow, and gives better results overall.

The most terrifying commit in this PR is 50775d0 which is a preparation to ea1b8a3 that implements getAnUltimateDefinition(). That's the actual predicate we use to go from *it to it = v.begin(), but in order to implement it properly I had to restructure quite a lot of the SSA predicates.

As a small bonus of 50775d0 it fixes a small amount of spurious flow and some inconsistency errors. The effect of this can be seen in 5f0efc1.

Finally, all of this leads up to 683fe26 which instantiates the shared SSA library to reason about iterators.

Commit-by-commit heavily recommended. I've split it up in (hopefully) meaningful chunks.