This works by adding data-flow edges to skip over array expressions when
reading from arrays. On the post-update side, there was already code to
skip over array expressions when storing to arrays. That happens in
`valueToUpdate` in `AddressFlow.qll`, which needed just a small tweak to
support assignments with non-field expressions at the top-level LHS,
like `*a = ...` or `a[0] = ...`.
The new code in `AddressFlow.qll` is copy-pasted from `EscapesTree.qll`,
and there is already a note in these files saying that they share a lot
of code and must be maintained in sync.
