Splits `ModuleVariableNode` away from `LocalSourceNode`, instead
creating a class `TypeTrackingNode` that encapsulates both of these.
This means we no longer have module variable nodes as part of
`LocalSourceNode` (which is good, since they have no "local" aspect to
them), and hence we can have `LocalSourceNode` inherit directly from
`ExprNode` (which makes the API a bit nicer).
Unfortunately these are breaking changes, so we can't actually fulfil
the above two desiderata until the `track` and `backtrack` methods on
`LocalSourceNode` have been fully deprecated. For this reason, we
preserve the present implementation of `LocalSourceNode`, and instead
lay the foundation for switching over in the future, by deprecating
`track` and `backtrack` on `LocalSourceNode`.
Going all the way to the AST layer seemed excessive to me, so I rewrote
it to do most of the logic at the data-flow layer. In principle this
_could_ result in more names being computed (due to splitting), but in
practice I don't expect this make a big difference.
This commit does a lot of stuff all at once, so here are the main
highlights:
In `TypeTracker.qll`, we change `StepSummary::step` to step only between
source nodes. Because reads and writes of global variables happen in two
different (jump) steps, this requires the intermediate
`ModuleVariableNode` to _also_ be a `LocalSourceNode`, and we therefore
modify the charpred for that class accordingly. (This also means
changing a few of the tests to account for these new source nodes.)
In addition, we change `TypeTracker::step` to likewise step between
local source nodes.
Next, to enable the use of the `track` convenience method on nodes, we
add some pragmas to `TypeTracker::step` that prevent bad joins from
occurring. With this, we can eliminate all of the manual type tracker
join predicates.
Next, we observe that because `StepSummary::step` now uses `flowsTo`, it
automatically encapsulates all local-flow steps. In particular this
means we do not have to use `typePreservingStep` in `smallstep`, but can
use `jumpStep` directly. A similar observation applies to
`TypeTracker::smallstep`.
Having done this, we no longer need `typePreservingStep`, so we get rid
of it.
