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

src/NadekoBot.Generators/Cloneable/SymbolExtensions.cs

@@ -0,0 +1,26 @@
+// Code temporarily yeeted from
+// https://github.com/mostmand/Cloneable/blob/master/Cloneable/CloneableGenerator.cs
+// because of NRT issue
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.CodeAnalysis;
+
+namespace Cloneable
+{
+    internal static class SymbolExtensions
+    {
+        public static bool TryGetAttribute(this ISymbol symbol, INamedTypeSymbol attributeType,
+            out IEnumerable<AttributeData> attributes)
+        {
+            attributes = symbol.GetAttributes()
+                .Where(a => SymbolEqualityComparer.Default.Equals(a.AttributeClass, attributeType));
+            return attributes.Any();
+        }
+
+        public static bool HasAttribute(this ISymbol symbol, INamedTypeSymbol attributeType)
+        {
+            return symbol.GetAttributes()
+                .Any(a => SymbolEqualityComparer.Default.Equals(a.AttributeClass, attributeType));
+        }
+    }
+}