MegaPhase#transformNode: add missing case for Thicket

A transformX method can return a Thicket of trees that need to be
transformed by subsequent phases, but so far this wasn't the case.

No testcase, but this is necessary to make the next commit work properly
as `ShortcutImplicits` returns a Thicket of DefDefs that need to be
transformed by TailRec

Author: smarter

compiler/src/dotty/tools/dotc/transform/MegaPhase.scala

@@ -156,7 +156,7 @@ class MegaPhase(val miniPhases: Array[MiniPhase]) extends Phase {
   private val cpy: TypedTreeCopier = cpyBetweenPhases
 
   /** Transform node using all phases in this group that have idxInGroup >= start */
-  def transformNode(tree: Tree, start: Int)(implicit ctx: Context) = {
+  def transformNode(tree: Tree, start: Int)(implicit ctx: Context): Tree = {
     def goNamed(tree: Tree, start: Int) = tree match {
       case tree: Ident => goIdent(tree, start)
       case tree: Select => goSelect(tree, start)
@@ -169,6 +169,8 @@ class MegaPhase(val miniPhases: Array[MiniPhase]) extends Phase {
     def goUnnamed(tree: Tree, start: Int) = tree match {
       case tree: Apply => goApply(tree, start)
       case tree: TypeTree => goTypeTree(tree, start)
+      case tree: Thicket =>
+        cpy.Thicket(tree)(tree.trees.mapConserve(transformNode(_, start)))
       case tree: This => goThis(tree, start)
       case tree: Literal => goLiteral(tree, start)
       case tree: Block => goBlock(tree, start)