Fix flag mutation for Scala2xPartiallyAugmented

Calling `setFlag` on the current denotation is not enough: there might
be other more recent denotations created, if we don't also set the flag
on these denotations, then the `is(Scala2xPartiallyAugmented)` check in
`LinkScala2Impls` will fail.

FIXME: we probably need to do the same for all the other usages of
setFlag/resetFlag/flags_= in the compiler.

Author: smarter

compiler/src/dotty/tools/dotc/transform/AugmentScala2Traits.scala

@@ -64,6 +64,6 @@ class AugmentScala2Traits extends MiniPhase with IdentityDenotTransformer { this
         || sym.isSuperAccessor) // scala2 superaccessors are pickled as private, but are compiled as public expanded
         sym.ensureNotPrivate.installAfter(thisPhase)
     }
-    mixin.setFlag(Scala2xPartiallyAugmented)
+    mixin.transformAfter(thisPhase, d => { d.setFlag(Scala2xPartiallyAugmented); d })
   }
 }

compiler/src/dotty/tools/dotc/transform/LinkScala2Impls.scala

@@ -60,7 +60,7 @@ class LinkScala2Impls extends MiniPhase with IdentityDenotTransformer { thisPhas
       if (needsMixinForwarder(sym) || sym.isConstructor || sym.isGetter && sym.is(Lazy) || sym.is(Method, butNot = Deferred))
         newImpl(sym.asTerm).enteredAfter(thisPhase)
     // The trait is now fully augmented so the flag isn't needed anymore.
-    mixin.resetFlag(Scala2xPartiallyAugmented)
+    mixin.transformAfter(thisPhase, d => { d.resetFlag(Scala2xPartiallyAugmented); d })
   }
 
   override def prepareForTemplate(impl: Template)(implicit ctx: Context): Context = {