C++: invoke unique without | |

Based on PR feedback. This will avoid a syntactic wart and make the
invocation do the right thing both with and without
`language[monotonicAggregates]`.

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

@@ -43,7 +43,7 @@ class Node extends TNode {
   /**
    * INTERNAL: Do not use. Alternative name for `getFunction`.
    */
-  final Function getEnclosingCallable() { result = unique( | | this.getFunction()) }
+  final Function getEnclosingCallable() { result = unique(Function f | f = this.getFunction() | f) }
 
   /** Gets the type of this node. */
   Type getType() { none() } // overridden in subclasses