Make constructor parameter $outer also Java synthetic

This does not work yet, since the backend does not issue a ACC_SYNTHETIC for
parameters labeled Artifact.

Author: odersky

compiler/src/dotty/tools/backend/jvm/BCodeHelpers.scala

@@ -343,6 +343,7 @@ trait BCodeHelpers extends BCodeIdiomatic with BytecodeWriters {
     def emitParamNames(jmethod: asm.MethodVisitor, params: List[Symbol]) =
       for param <- params do
         var access = asm.Opcodes.ACC_FINAL
+        if param.is(Artifact) then access |= asm.Opcodes.ACC_SYNTHETIC
         jmethod.visitParameter(param.name.mangledString, access)
 
     /*

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

@@ -929,7 +929,7 @@ object Erasure {
       if constr.isConstructor && needsOuterParam(constr.owner.asClass) then
         constr.info match
           case MethodTpe(outerName :: _, outerType :: _, _) =>
-            val outerSym = newSymbol(constr, outerName, Flags.Param, outerType)
+            val outerSym = newSymbol(constr, outerName, Flags.Param | Flags.SyntheticArtifact, outerType)
             ValDef(outerSym) :: Nil
           case _ =>
             // There's a possible race condition that a constructor was looked at

tests/run/i14083.scala

@@ -2,6 +2,10 @@ class Outer {
   class Inner
 }
 @main def Test =
+  assert(classOf[Outer#Inner]
+    .getConstructors.head
+    .getParameters.head
+    .isSynthetic)
   assert(
     classOf[Outer#Inner]
       .getDeclaredFields