Avoid leaking untyped trees out of macro

The stack trace and bisection in #119 made me notice that we
are failing to typecheck the cast we generated in the fix for #74.

The ticket doesn't have a reproduction, so I'm submitting this
without a test case.

Fixes #119

(cherry picked from commit a037da1)

Author: retronym

src/main/scala/scala/async/internal/AnfTransform.scala

@@ -81,7 +81,7 @@ private[async] trait AnfTransform {
             // TODO avoid creating a ValDef for the result of this await to avoid this tree shape altogether.
             // This will require some deeper changes to the later parts of the macro which currently assume regular
             // tree structure around `await` calls.
-              gen.mkCast(ref, definitions.UnitTpe)
+              localTyper.typedPos(tree.pos)(gen.mkCast(ref, definitions.UnitTpe))
             else ref
             stats :+ valDef :+ atPos(tree.pos)(ref1)