Write the generated file header to the ComInterfaceGenerator-generated files (#86283)

Author: jkoritzinsky

src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/ComInterfaceGenerator.cs

@@ -183,6 +183,7 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
                     var ((((((interfaceContext, interfaceInfo), managedToNativeStubs), nativeToManagedStubs), nativeToManagedVtable), iUnknownDerivedAttribute), shadowingMethod) = data;
 
                     using StringWriter source = new();
+                    source.WriteLine("// <auto-generated />");
                     interfaceInfo.WriteTo(source);
                     // Two newlines looks cleaner than one
                     source.WriteLine();