C#: Support postupdate notes for suppress nullable warning expressions.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/dataflow/internal/DataFlowPrivate.qll

@@ -420,7 +420,8 @@ module LocalFlow {
       e instanceof ConditionalExpr or
       e instanceof Cast or
       e instanceof NullCoalescingExpr or
-      e instanceof SwitchExpr
+      e instanceof SwitchExpr or
+      e instanceof SuppressNullableWarningExpr
     )
   }