Better marking of internal calls

[marek-safar]

The linker today marks internal-calls layout dependencies the same way as pinvokes. This is not needed and overmarks more than is required. The logic needs to be updated to treat internal-calls differently.

The relevant logic starts at https://github.com/mono/linker/blob/master/src/linker/Linker.Steps/MarkStep.cs#L2491

[marek-safar]

@MichalStrehovsky I assume CoreCLR does not need special handling either right?

[MichalStrehovsky]

I think we fully preserve all the types that the runtime relies on through the generated ILLinkTrim in CoreCLR's CoreLib.