Cast the while(true) loop to Nothing instead of throw null.

In fact, since we are after erasure, we have to materialize a
`BoxedUnit.UNIT`, and cast that to `Nothing`.

Author: sjrd

compiler/src/dotty/tools/dotc/transform/TailRec.scala

@@ -75,7 +75,7 @@ import reporting.diagnostic.messages.TailrecNotApplicable
  *        }
  *      }
  *    }
- *    throw null // unreachable
+ *    BoxedUnit.UNIT.asInstanceOf[Nothing] // unreachable
  *  }
  *  }}}
  *
@@ -170,7 +170,7 @@ class TailRec extends MiniPhase {
                 })
               }) ::
               Nil,
-              Throw(Literal(Constant(null))) // unreachable code
+              ref(defn.BoxedUnit_UNIT).asInstance(defn.NothingType) // unreachable code
             )
           } else {
             if (mandatory) ctx.error(