C++: Add an abstract class that can be used to extend 'viableCallable'.

cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowDispatch.qll

@@ -27,6 +27,9 @@ DataFlowCallable viableCallable(DataFlowCall call) {
   or
   // Virtual dispatch
   result = call.(VirtualDispatch::DataSensitiveCall).resolve()
+  or
+  // Additional call targets
+  result = any(AdditionalCallTarget additional).viableTarget(call.getUnconvertedResultExpression())
 }
 
 /**

cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll

@@ -14,6 +14,7 @@ private import DataFlowPrivate
 private import ModelUtil
 private import SsaInternals as Ssa
 private import DataFlowImplCommon as DataFlowImplCommon
+private import codeql.util.Unit
 
 /**
  * The IR dataflow graph consists of the following nodes:
@@ -2237,3 +2238,15 @@ module InstructionBarrierGuard<instructionGuardChecksSig/3 instructionGuardCheck
     )
   }
 }
+
+/**
+ * A unit class for adding additional call steps.
+ *
+ * Extend this class to add additional call steps to the data flow graph.
+ */
+class AdditionalCallTarget extends Unit {
+  /**
+   * Gets a viable target for `call`.
+   */
+  abstract DataFlowCallable viableTarget(Call call);
+}