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, 1 insertions, 0 deletions

diff --git a/test/files/pos/anyval-rangepos.flags b/test/files/pos/anyval-rangepos.flags
new file mode 100644
index 0000000000..fcf951d907
--- /dev/null
+++ b/test/files/pos/anyval-rangepos.flags
@@ -0,0 +1 @@
+-Yrangepos
\ No newline at end of file