C#: Add support for ref readonly parameters in the stub generator.

Author: michaelnebel

csharp/extractor/Semmle.Extraction.CSharp.StubGenerator/StubVisitor.cs

@@ -534,6 +534,9 @@ private void StubParameters(ICollection<IParameterSymbol> parameters)
                 case RefKind.In:
                     stubWriter.Write("in ");
                     break;
+                case RefKind.RefReadOnlyParameter:
+                    stubWriter.Write("ref readonly ");
+                    break;
                 default:
                     stubWriter.Write($"/* TODO: {parameter.RefKind} */");
                     break;
@@ -884,4 +887,4 @@ public override void VisitProperty(IPropertySymbol symbol)
         if (explicitInterfaceImplementations.Length == 0)
             StubProperty(symbol, null);
     }
-}
+}