Ruby: Fix isLocalSourceNode implementation

The old code was equivalent with the code below, which seems wrong

```
not n instanceof ExprNode
or
n instanceof ExprNode and
localFlowStepTypeTracker+(..., n)
```

From running on real DB I found that this meant that the following node
types were also included as local source nodes:

- `TReturningNode`
- `TSynthReturnNode`
- `TSummaryNode`
- `TSsaDefinitionNode`

My understanding is that the first 3 should not be included.

I would guess that SsaDefinitionNode should indeed be included as a
LocalSourceNode, but I'm not 100% sure, so I'll see what the test
results say before making further changes.

Author: RasmusWL

ruby/ql/lib/codeql/ruby/dataflow/internal/DataFlowPrivate.qll

@@ -303,11 +303,12 @@ private module Cached {
     n instanceof PostUpdateNodes::ExprPostUpdateNode
     or
     // Expressions that can't be reached from another entry definition or expression.
+    n instanceof ExprNode and
     not localFlowStepTypeTracker+(any(Node n0 |
         n0 instanceof ExprNode
         or
         entrySsaDefinition(n0)
-      ), n.(ExprNode))
+      ), n)
     or
     // Ensure all entry SSA definitions are local sources -- for parameters, this
     // is needed by type tracking. Note that when the parameter has a default value,