C#: Update guards library implementation.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/controlflow/Guards.qll

@@ -907,7 +907,7 @@ module Internal {
     or
     // "In string concatenation operations, the C# compiler treats a null string the same as an empty string."
     // (https://docs.microsoft.com/en-us/dotnet/csharp/how-to/concatenate-multiple-strings)
-    e instanceof AddExpr and
+    e instanceof AddOperation and
     e.getType() instanceof StringType
     or
     e.(DefaultValueExpr).getType().isValueType()