Add @sharable

Author: odersky

compiler/src/dotty/tools/dotc/cc/CaptureSet.scala

@@ -1130,6 +1130,7 @@ object CaptureSet:
     override def frozen = Frozen.All
     override def putHidden(v: HiddenSet, elems: Refs): Boolean = false
 
+  @sharable
   /** A frozen state that allows a Fresh.Cap instancce to subsume a
    *  reference `r` only if `r` is already present in the hidden set of the instance.
    *  No new references can be added.