Use Type.GetTypeCode() instead of private extension

[StephenMolloy]

Use Type.GetTypeCode instead of private extension.

In the 1.0 days, Type.GetTypeCode() did not exist. So we created our
own extension method and used that. Now that this method exists
in .Net Core again, we should use the framework version instead
of our own. Doing so fixes edge case issues like #53553.

Also, while adding a test that checks for the regression
reported in #53553, I found that the reflection-based serializer
was taking a logically different approach to serialization when
writing out an enum (or array) that has been cast down (to object
in this case.) This PR also brings the reflection-based serializer
in line with how the refemit logic.

Fixes #53553