This commit removes SSA nodes from the data flow graph. Specifically, for a definition and use such as
```python
x = expr
y = x + 2
```
we used to have flow from `expr` to an SSA variable representing x and from that SSA variable to the use of `x` in the definition of `y`. Now we instead have flow from `expr` to the control flow node for `x` at line 1 and from there to the control flow node for `x` at line 2.
Specific changes:
- `EssaNode` from the data flow layer no longer exists.
- Several glue steps between `EssaNode`s and `CfgNode`s have been deleted.
- Entry nodes are now admitted as `CfgNodes` in the data flow layer (they were filtered out before).
- Entry nodes now have a new `toString` taking into account that the module name may be ambigous.
- Some tests have been rewritten to accomodate the changes, but only `python/ql/test/experimental/dataflow/basic/maximalFlowsConfig.qll` should have semantic changes.
- Comments have been updated
- Test output has been updated, but apart from `python/ql/test/experimental/dataflow/basic/maximalFlows.expected` only `python/ql/test/experimental/dataflow/typetracking-summaries/summaries.py` should have a semantic change. This is a bonus fix, probably meaning that something was never connected up correctly.
Also needed to fix up `TestUtil/UnresolvedCalls.qll` after a bad merge
conflict resolution. Since all calls are now DataFlowCall, and not JUST
the ones that can be resolved, we need to put in the restriction that
the callable can also be resolved.
- still identifying summarized callables by name.
I think ther shoudl perhaps be a `getAUse` next to `getACall`.
- also fix tests, adding a standard taint configuration
Two classes have been inserted into the hierarchies:
- `NonLibraryDataFlowCallable` with a method `getACall2`.
This method implements "get a call, not considering flow summaries".
For `NonLibraryDataFlowCallable`s, `getACall` will defer to `getACall2`.
While you could have a synthesised call to such a callable,
it would not correspond to a `CallNode`.
- `NonLibraryDataFlowSourceCall` with methods
`getArg2` and `getCallable2`. These also refer to a call graph that
does not consider flow summaries.
`getArg2` is used to synthesise pre-update nodes for arguments.
`getCallable2` is used in `connects` to compute argument passing.
This is used to define data flow nodes for overflow arguments.
`getACall2` ensures that `LibraryCallableValue::getACall` is not called
when the charpred of `FunctionCall` is evaluated.
