New option -Ypos-debug, and fixed range position breakage.

(Looks like there is more range position breakage yet, but
this gets the outermost layer.)

Channeling my struggles into a slightly easier future.

  % scalac -Ypos-debug -d /tmp ./src/library/scala/Predef.scala
  ./src/library/scala/Predef.scala:222: warning:  Positioned tree has unpositioned child in phase extmethods
      def x = __resultOfEnsuring
              ^
  parent: #7109  line 222  Select     // (value __resultOfEnsuring in class Ensuring)
   child: #7108            Ident      // (value $this)

  ./src/library/scala/Predef.scala:258: warning:  Positioned tree has unpositioned child in phase extmethods
      def x = __leftOfArrow
              ^
  parent: #7280  line 258  Select     // (value __leftOfArrow in class ArrowAssoc)
   child: #7279            Ident      // (value $this)

  two warnings found

Or try this to really see some output:

  % scalac -Yrangepos -Ypos-debug

1 files changed, 4 insertions, 5 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/AddInterfaces.scala b/src/compiler/scala/tools/nsc/transform/AddInterfaces.scala
index 555d0700ae..39e2cbe694 100644
--- a/src/compiler/scala/tools/nsc/transform/AddInterfaces.scala
+++ b/src/compiler/scala/tools/nsc/transform/AddInterfaces.scala
@@ -264,11 +264,10 @@ abstract class AddInterfaces extends InfoTransform { self: Erasure =>
     else DefDef(clazz.primaryConstructor, Block(List(), Literal(Constant()))) :: stats
 
   private def implTemplate(clazz: Symbol, templ: Template): Template = atPos(templ.pos) {
-    val templ1 = atPos(templ.pos) {
-      Template(templ.parents, emptyValDef,
-               addMixinConstructorDef(clazz, templ.body map implMemberDef))
-        .setSymbol(clazz.newLocalDummy(templ.pos))
-    }
+    val templ1 = (
+      Template(templ.parents, emptyValDef, addMixinConstructorDef(clazz, templ.body map implMemberDef))
+        setSymbol clazz.newLocalDummy(templ.pos)
+    )
     templ1.changeOwner(templ.symbol.owner -> clazz, templ.symbol -> templ1.symbol)
     templ1
   }