C++: Rewrite reachable without mutual recursion

[jbj]

This gives a measurable speedup, and I think it makes the code clearer. Before this, the reachable and successors_adapted predicates were mutually recursive together with four other predicates over two files.

I measured these benchmarks, where the numbers came from subtracting timestamps in the log rather than trusting the "Clause timing report" at the end:

• wireshark: reduced from 204s to 121s.
• linux: reduced from 59s to 36s.
• postgres: reduced from 28s to 12s.

I tested correctness and performance with this query, which gives the same results before and after:

select strictcount(ControlFlowNode node | reachable(node))

These changes were supposed to be a refactoring to enable even more ambitious speedups of the reachable predicate, but they bring large enough improvements on their own that I'd like to submit them as a separate PR. It's not clear whether it's worth following up with changes to make the code faster but more complex.

The performance issue with reachable is that it has a recursive occurrence on both sides of an and. That was hidden before but is visible after this refactoring. It's more expensive to compute such a general case of recursion than the usual case where one side of every and is constant.

[jbj]

The test failure looks like a real problem. I'll look into it.

[jbj]

Notes to self:

The successors_adapted can't work the way I've written it here. It has to check pred, not succ for the sake of code like if (...) { aborting() } else { not_aborting() } next_stmt;. Maybe it should use the aborting predicate.

Since aborting and abortingFunction are not cached, might this cause ConstantExprs.qll to be recomputed in the BasicBlocks stage under memory pressure? That's both before and after this PR.

[jbj]

This branch was not quite ready for PR. Closing.