Update annotations for implementations of ILabelSymbol

src/Compilers/CSharp/Portable/Symbols/PublicModel/LabelSymbol.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
-using System.Diagnostics;
+#nullable enable
+
+using System.Diagnostics.CodeAnalysis;
+using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.CSharp.Symbols.PublicModel
 {
@@ -10,7 +13,7 @@ internal sealed class LabelSymbol : Symbol, ILabelSymbol
 
         public LabelSymbol(Symbols.LabelSymbol underlying)
         {
-            Debug.Assert(underlying is object);
+            RoslynDebug.Assert(underlying is object);
             _underlying = underlying;
         }
 
@@ -31,6 +34,7 @@ protected override void Accept(SymbolVisitor visitor)
             visitor.VisitLabel(this);
         }
 
+        [return: MaybeNull]
         protected override TResult Accept<TResult>(SymbolVisitor<TResult> visitor)
         {
             return visitor.VisitLabel(this);