C++: Override toString on argument indirections

Without this override, end users would see the string
`BufferReadSideEffect` in path explanations.

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

@@ -387,6 +387,20 @@ class DefinitionByReferenceNode extends InstructionNode {
   }
 }
 
+/**
+ * A node representing the memory pointed to by a function argument.
+ *
+ * This class exists only in order to override `toString`, which would
+ * otherwise be the default implementation inherited from `InstructionNode`.
+ */
+private class ArgumentIndirectionNode extends InstructionNode {
+  override ReadSideEffectInstruction instr;
+
+  override string toString() {
+    result = "Argument " + instr.getIndex() + " indirection"
+  }
+}
+
 /**
  * A `Node` corresponding to a variable in the program, as opposed to the
  * value of that variable at some particular point. This can be used for