Adds API graph support for observing that in
```python
def foo(x : Bar): ...
```
The variable `x` is likely to be an instance of the type `Bar` inside
this function.
In particular, we add `getInstanceFromAnnotation` as a predicate on API
graph nodes that tracks this step (corresponding to a new edge type
labeled with "annotation" in the API graph), and extend the existing
`getAnInstance` predicate to also include instances arising from type
annotations.
A more complete solution would also add support for annotated
assignments (`x : Foo = ...` or just `x : Foo`) as well as track types
through type aliases (`type Foo = Bar`). This turns out to be
non-trivial, however, as these type constructs don't have any CFG nodes
(and so no data-flow nodes by default either). In order to not have
perfect be the enemy of good, this commit is only targeting the type
parameter case (which is also likely to be the most common use case
anyway).
The tests for API graphs have been extended accordingly, including tests
for the kinds of type ascriptions that we _don't_ currently model in API
graphs (marked with `MISSING:` in the inline tests).
