Fix test for decompilation

Otherwise the typer will infer the type `wr.underlying.T`, which is not valid in source:

    val x: wr.underlying.T = wr.underlying.get()

Author: liufengyun

tests/pos/i1793.scala

@@ -1,7 +1,7 @@
 object Test {
   import scala.ref.WeakReference
   def unapply[T <: AnyRef](wr: WeakReference[T]): Option[T] = {
-    val x = wr.underlying.get
+    val x: T = wr.underlying.get
     if (x != null) Some(x) else None
   }
 }