Enabled Nullable reference types. Added a temporary fix for clonable NRT warnings.

src/NadekoBot.Generators/Cloneable/SyntaxReceiver.cs

@@ -0,0 +1,27 @@
+// Code temporarily yeeted from
+// https://github.com/mostmand/Cloneable/blob/master/Cloneable/CloneableGenerator.cs
+// because of NRT issue
+using System.Collections.Generic;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+
+namespace Cloneable
+{
+    internal class SyntaxReceiver : ISyntaxReceiver
+    {
+        public IList<ClassDeclarationSyntax> CandidateClasses { get; } = new List<ClassDeclarationSyntax>();
+
+        /// <summary>
+        /// Called for every syntax node in the compilation, we can inspect the nodes and save any information useful for generation
+        /// </summary>
+        public void OnVisitSyntaxNode(SyntaxNode syntaxNode)
+        {
+            // any field with at least one attribute is a candidate for being cloneable
+            if (syntaxNode is ClassDeclarationSyntax classDeclarationSyntax &&
+                classDeclarationSyntax.AttributeLists.Count > 0)
+            {
+                CandidateClasses.Add(classDeclarationSyntax);
+            }
+        }
+    }
+}