Restore BundleUpcastable (#3487) (#3494)

(cherry picked from commit b904c19)

Co-authored-by: Jiuyang Liu <liu@jiuyang.me>

core/src/main/scala/chisel3/experimental/dataview/package.scala

@@ -37,6 +37,17 @@ package object dataview {
     }
   }
 
+  /** Provides `viewAsSupertype` for subclasses of [[Bundle]] */
+  @deprecated("Use RecordUpcastable instead", "chisel3.6")
+  implicit class BundleUpcastable[T <: Bundle](target: T) {
+
+    /** View a [[Bundle]] or [[Record]] as a parent type (upcast) */
+    def viewAsSupertype[V <: Bundle](proto: V)(implicit ev: ChiselSubtypeOf[T, V], sourceInfo: SourceInfo): V = {
+      implicit val dataView = PartialDataView.supertype[T, V](_ => proto)
+      target.viewAs[V]
+    }
+  }
+
   /** Provides `viewAsSupertype` for subclasses of [[Record]] */
   implicit class RecordUpcastable[T <: Record](target: T) {