Fix scala#4060 by marking ghost symbols as unstable

Author: Blaisorblade

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -600,7 +600,7 @@ object SymDenotations {
 
     /** Is this a denotation of a stable term (or an arbitrary type)? */
     final def isStable(implicit ctx: Context) =
-      isType || is(Stable) || !(is(UnstableValue) || info.isInstanceOf[ExprType])
+      !is(Ghost) && (isType || is(Stable) || !(is(UnstableValue) || info.isInstanceOf[ExprType]))
 
     /** Is this a "real" method? A real method is a method which is:
      *  - not an accessor