classIndexIndex only indexes the genericInstance, not the actual generic
instance type. Therefore, for example, A<Int32,Int32> and B<Int32,Int32>
have the same classIndexIndex because the generic parameters are the
same, despite being otherwise unrelated.
Instead of TypesByMethodSpecClassIndex, we simply call MakeGenericType
each time, relying on genericTypeInstances to dedup the resulting
instances. This patch thus also adds all of the types from
genericTypeInstances to the Il2CppModel.Types listing.
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