Allow uncheckedStable on defs, vars

This can be handy for retaining source compatibility. Or for when you
just know better.

Fixes scala/bug#8908

Author: lrytz

library/src/scala/annotation/unchecked/uncheckedStable.scala

@@ -12,7 +12,10 @@
 
 package scala.annotation.unchecked
 
+import scala.annotation.meta.{field, getter}
+
 /** An annotation for values that are assumed to be stable even though their
  *  types are volatile.
  */
+@getter @field
 final class uncheckedStable extends scala.annotation.StaticAnnotation {}