Fix wrong period for creating nested ctx in TreeTransforms.

`owner` for nestedContext in `TreeTransforms` was assigned
using initial period of whole `TreeTransforms` block.
That is incorrect as denotation for this owner could have been
create by some phase in this block.

Author: DarkDimius

src/dotty/tools/dotc/transform/TreeTransform.scala

@@ -1130,10 +1130,13 @@ object TreeTransforms {
 
     def transform(tree: Tree, info: TransformerInfo, cur: Int)(implicit ctx: Context): Tree = ctx.traceIndented(s"transforming ${tree.show} at ${ctx.phase}", transforms, show = true) {
       if (cur < info.transformers.length) {
+        // if cur > 0 then some of the symbols can be created by already performed transformations
+        // this means that their denotations could not exists in previous periods
+        val pctx = ctx.withPhase(info.transformers(cur))
         tree match {
           //split one big match into 2 smaller ones
-          case tree: NameTree => transformNamed(tree, info, cur)
-          case tree => transformUnnamed(tree, info, cur)
+          case tree: NameTree => transformNamed(tree, info, cur)(pctx)
+          case tree => transformUnnamed(tree, info, cur)(pctx)
         }
       } else tree
     }