Preserve assemblies when referenced through reflection with missing type

@sbomer in https://github.com/mono/linker/issues/1164#issuecomment-723368973:

> What behavior do we want for a dynamically loaded assembly that's unused? For example, GetType("MissingType, MyAssembly") would load MyAssembly without requiring code from it. This won't run module initializers (at least on coreclr), but my reading of the spec is that implementations could run module initializers as soon as the module is loaded.
>
> The current behavior allows the linker to remove MyAssembly. Assuming this is what we want, what should happen if the default action is copy? Normally, copy will keep unused statically referenced assemblies - but it's not clear to me what should happen for unused dynamic references.

@MichalStrehovsky in https://github.com/mono/linker/issues/1164#issuecomment-724064804

> GetType("MissingType, MyAssembly", throwOnError: true) throws different exceptions based on whether the file, or the type doesn't exist. Arguably this is very niche, but I would just keep the assembly so that we get this variation right, unless it's too much work (more than 5 minutes :)) to do that (I assume it really doesn't need much work - if we have trouble keeping assemblies with nothing in it marked, we can just mark the "" type so that we have something marked for sure and keeping the assembly will probably just fall out).