Remove exclusion of DebuggerTypeProxyAttribute (#251)

***NO_CI***

Author: josesimoes

nanoFramework.CoreLibrary/System/Diagnostics/DebuggerAttributes.cs

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Runtime.CompilerServices;
@@ -244,7 +244,6 @@ public DebuggerBrowsableState State
     /// </summary>
     /// <remarks>Available only in mscorlib build with support for System.Reflection.</remarks>
     [AttributeUsage(AttributeTargets.Struct | AttributeTargets.Class | AttributeTargets.Assembly, AllowMultiple = true)]
-    [ExcludeType]
     public sealed class DebuggerTypeProxyAttribute : Attribute
     {
         private readonly string _typeName;