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, 2 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
index ab6125df61..ccebcfa54d 100644
--- a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
+++ b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
@@ -394,7 +394,7 @@ self =>
 
       // object Main
       def moduleName  = newTermName(ScriptRunner scriptMain settings)
-      def moduleBody  = Template(List(scalaAnyRefConstr), emptyValDef, List(emptyInit, mainDef))
+      def moduleBody  = Template(List(atPos(o2p(in.offset))(scalaAnyRefConstr)), emptyValDef, List(emptyInit, mainDef))
       def moduleDef   = ModuleDef(NoMods, moduleName, moduleBody)
 
       // package <empty> { ... }
@@ -2738,7 +2738,7 @@ self =>
       def anyrefParents() = {
         val caseParents = if (mods.isCase) List(productConstr, serializableConstr) else Nil
         parents0 ::: caseParents match {
-          case Nil  => List(scalaAnyRefConstr)
+          case Nil  => List(atPos(o2p(in.offset))(scalaAnyRefConstr))
           case ps   => ps
         }
       }