In `x with { Foo = bar }`, instead of having a single data-flow step
`x => x with { Foo = bar }`
we now have two steps:
`x => { Foo = bar }`
and
`{ Foo = bar } => x with { Foo = bar }`
Moreover, `clearsContent` now targets the object initializer instead of the
whole `with` expression, which means that it will only apply to values carried
over from the old object and not those explicitly stored into the new object.
- Introduce `ReadTaintNode` and `TaintStoreNode` to simplify logic for taint
getters and taint setters, respectively.
- `nodeCandFwd2`: Restrict `stored` column after a read, based on what it might
be before a store of the same field.
- `nodeCand2`: Restrict `read` column (renamed from `stored`) after a store, based
on what it might be after a read of the same field.
- Move big step predicates into a `LocalFlowBigStep` module.
- Define predicates by dispatch in `AccessPath[Front]` class.
- `flowCandFwd0`: Restrict `apf` column after a read, as it should be able to match
a Boolean `read` column from `nodeCand2`.
- `flowFwd0`: Restrict columns `ap` and `apf` after a read, by introducing a
`flowConsCandFwd` predicate (similar to what is done in the previous pruning steps).
- `flowFwd0`: Restrict columns `ap` and `apf` after a store, by introducing a
`flowConsCand` predicate (similar to what is done in the previous pruning steps).
The predicate
```
argumentValueFlowsThrough(ArgumentNode arg, OutNode out, CallContext cc)
```
has been generalized to
```
argumentValueFlowsThrough(
DataFlowCall call, ArgumentNode arg, Node out, ContentOption contentIn,
ContentOption contentOut
)
```
This enables us to summarize normal flow-through (as before), getters, setters,
as well as getter-setters.
This commit adds type information to data flow paths, by mapping node types onto
the smaller set of GVN types, and implementing `ppReprType()`.
The effect is a mere change in `DataFlow::PathNode::toString()`; no type-based
pruning is done yet.
