Override removal

[mrvoorhe]

If no instance of a type can exist, then it's safe to not mark overrides as long as the method being overridden is not abstract.

This change should be good to go. I've added WIP as I'm still running these changes through our il2cpp test suite as that is a valuable way to get some extra coverage on linker changes.

Do not land this branch before #454 otherwise there will be problems.

[mrvoorhe]

@marek-safar FYI for perspective this PR is not a particularly big size for a Console.WriteLine("Hello world") program. IT only saves 0.06%. It does better in other scenarios, for example, in the artificial typeof(System.Xml.XmlDocument) scenario in #455 this PR is able to get the same size savings.

[mrvoorhe]

Fixed the test failure. EmbeddedLinkXmlPreservesAdditionalAssemblyWithOverriddenMethod needed to be updated to mark a .ctor on some types.

I still need to check on the il2cpp test results w/ this change before I drop the WIP

[mrvoorhe]

@marek-safar This PR is ready for final review. After preserving the .ctor() of InternalThread our il2cpp test suites were green. I also preserved the .ctor() of MonoListItem because looking at our mono runtime code I found that it should be, but that one doesn't seem to have been as critical as the InternalThread.

Do you want me to open a PR to mono master adding either of these .ctor() preserves to mscorlib.xml? You had seemed to imply that InternalThread has been renamed?

[mrvoorhe]

Looks like after rebasing and picking up the Finalize() fix I have a couple test failures. I will take a look.

[mrvoorhe]

Easy fix. Needed to update the NeverInstantiatedTypeWithOverridesFromObject tests to no longer assert that the overrides from System.Object are kept 😄