[C++] Fix union copy constructor to work with nested structs (#6552)

[piotrdz]

This will assert that any struct type is copyable and the corresponding copy constructor call will be generated.

For table types, the logic remains unchanged.

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[piotrdz]

@googlebot I signed it!

[aardappel]

Well, that is simple.. I guess there were no existing tests affected by this? did you run generate_code.sh ?

[piotrdz]

I ran generate_code.sh, but there were no changes to the generated files. This pattern (union of structs) is not used in tests.

[aardappel]

Ok, fair enough, we'll merge this for now.