If we allow JetLightClass to be created on built-in types, a normal codegen
would be launched, with a mapping from Kotlin to Java types enabled, every time
IDE requests built-in classes. Codegen would then try to make something
meaningless, e.g. create a class for a primitive type (since our jet.Boolean is
mapped to primitive boolean). This would result in different exceptions when
navigating to built-in library from IDE.
Add a helpful error message to ClassFileFactory if we ever again produce
classes for primitive types.
Alexander Udalov