Fix S3260: seal private classes that have no derived types.

Author: tillig

src/Autofac/Core/Registration/ExternalComponentRegistration.cs

@@ -27,7 +27,7 @@ protected override IResolvePipeline BuildResolvePipeline(IComponentRegistryServi
         return Target.ResolvePipeline;
     }
 
-    private class NoOpActivator : IInstanceActivator
+    private sealed class NoOpActivator : IInstanceActivator
     {
         public NoOpActivator(Type limitType)
         {

src/Autofac/Diagnostics/DefaultDiagnosticTracer.cs

@@ -234,7 +234,7 @@ protected override void OnOperationSuccess(OperationSuccessDiagnosticData data)
     /// <summary>
     /// Provides a string builder that auto-indents lines.
     /// </summary>
-    private class IndentingStringBuilder
+    private sealed class IndentingStringBuilder
     {
         private const int IndentSize = 2;