C++: change note for new FoldExpr class

Author: nickrolfe

change-notes/1.21/analysis-cpp.md

@@ -33,3 +33,4 @@
 - Additional support for definition by reference has been added to the `semmle.code.cpp.dataflow.TaintTracking` library.
     - The taint tracking library now includes taint-specific edges for functions modeled in `semmle.code.cpp.models.interfaces.DataFlow`.
     - The taint tracking library adds flow through library functions that are modeled in `semmle.code.cpp.models.interfaces.Taint`. Queries can add subclasses of `TaintFunction` to specify additional flow.
+- There is a new `FoldExpr` class, representing C++17 fold expressions.