Use children operation in Space

Now that we have defined in centrally, we can avoid the duplication
in Space.

Author: odersky

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -514,12 +514,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
 
   /** Decompose a type into subspaces -- assume the type can be decomposed */
   def decompose(tp: Type): List[Space] = {
-    val children = tp.classSymbol.annotations.filter(_.symbol == ctx.definitions.ChildAnnot).map { annot =>
-      // refer to definition of Annotation.makeChild
-      annot.tree match {
-        case Apply(TypeApply(_, List(tpTree)), _) => tpTree.symbol
-      }
-    }
+    val children = tp.classSymbol.children
 
     debug.println(s"candidates for ${tp.show} : [${children.map(_.show).mkString(", ")}]")