Make outer references Java-synthetic

Fixes #14083

I verified manually that the puter accessors are now ACC_SYNHETIC. If someone wants to add a bytecode test, this would be good. I don't know how to make one.

Author: odersky

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

@@ -177,7 +177,7 @@ object ExplicitOuter {
           else prefix.widen)
     val info = if (flags.is(Method)) ExprType(target) else target
     atPhaseNoEarlier(explicitOuterPhase.next) { // outer accessors are entered at explicitOuter + 1, should not be defined before.
-      newSymbol(owner, name, Synthetic | flags, info, coord = cls.coord)
+      newSymbol(owner, name, SyntheticArtifact | flags, info, coord = cls.coord)
     }
   }
 

tests/pos/i14083.scala

@@ -0,0 +1,3 @@
+class Outer {
+  class Inner
+}