Make sure the trimmable static registrar doesn't modify user assemblies

In order to support Hot Reload, we can't modify user assemblies.

Unfortunately our non-dynamic registrars are designed around modifying assemblies... so we need to make sure there's a way to not modify assemblies when using the trimmable static registrar (we only care about this one + the dynamic one, because all the other static registrars are going to be unnecessary and eventually removed).

Task:

* [ ] Identify steps that modify user assemblies.
* [ ] PreserveSmartEnumConversionStep is one: https://github.com/dotnet/macios/blob/a92fbe4b82814678d90847303735fc1df41f1863/tools/dotnet-linker/PreserveSmartEnumConversionsStep.cs#L37-L38
* [ ] Others?
* [ ] Design a solution for each case.
* [ ] Implement said solutions.

Start with PreserveSmartEnumConversionsStep.cs at the linked lines and trace how the trimmable static registrar processes user assemblies. Identify other steps that modify those assemblies, then review the registrar and linker entry points involved. Done means each modification path has a designed and implemented alternative that preserves user assemblies for Hot Reload.

Written by the indexing model from the issue text.