C++: use explicit this

Author: nickrolfe

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

@@ -229,7 +229,7 @@ class Node extends TIRDataFlowNode {
   Expr asIndirectArgument() { result = this.asIndirectArgument(_) }
 
   /** Gets the positional parameter corresponding to this node, if any. */
-  Parameter asParameter() { result = asParameter(0) }
+  Parameter asParameter() { result = this.asParameter(0) }
 
   /**
    * Gets the uninitialized local variable corresponding to this node, if