C++: Add back getSubBasicBlockStart

It turns out this predicate was used in a test, and that use can't be
replaced with the new `partiallyDefinesVariableAt` predicate since
`partiallyDefinesVariableAt` doesn't hold for a `PartialDefinition` that
defines something other than a variable.

cpp/ql/src/semmle/code/cpp/dataflow/internal/FlowVar.qll

@@ -124,6 +124,11 @@ private module PartialDefinitions {
 
     deprecated predicate partiallyDefinesThis(ThisExpr e) { innerDefinedExpr = e }
 
+    /**
+     * Gets the subBasicBlock where this `PartialDefinition` is defined.
+     */
+    ControlFlowNode getSubBasicBlockStart() { result = node }
+
     /**
      * Holds if this `PartialDefinition` defines variable `v` at control-flow
      * node `cfn`.