SI-8574 Copy @SerialVersionUID, etc, to specialized subclasses

The test case demonstrates that this is important for serialization
and for strictfp. (Although the latter is still pretty broken,
see SI-7954.)

Now that the synthetic subclass of `Tuple2[Int, Int]` also has the
`@deprecatedInheritance` annotation, I had to change the spot that
issues this warning to be silent after the typer phase. Otherwise,
we get two warnings in `run/t3888.scala`. This also remedies double
warnings that were incurred in `neg/t6162-inheritance`.