Constructor companion gets privateWithin

[som-snytt]

Follow-up to #22560
Fixes #22626

When creating a companion for a constructor proxy, the companion is protected if the class is. Now it also receives the privateWithin boundary of the class, for protected[p].

[som-snytt]

Confirmed that hkd4s compiles again, and also that it compiles using

-protected[tschuchort] class ArrayProduct(val elems: Array[Any]) extends Product:
+private[tschuchort] class ArrayProduct(val elems: Array[Any]) extends Product:

What other semantics might be expected?