Fix #3476: Add Artifact flag to closures

This way, we will not generate generic java signatures for closures.

Note that scalac also marks closures as `Artifact` during `UnCurry`.

See:
 - https://github.com/scala/scala/blob/7c8e4ac0797872190f67d1738251ded1980d8d77/src/compiler/scala/tools/nsc/ast/TreeGen.scala#L260-L272
 - https://github.com/scala/scala/blob/7c8e4ac0797872190f67d1738251ded1980d8d77/src/compiler/scala/tools/nsc/transform/UnCurry.scala#L221-L222

Author: Duhemm

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -805,7 +805,7 @@ object desugar {
    *  If `inlineable` is true, tag $anonfun with an @inline annotation.
    */
   def makeClosure(params: List[ValDef], body: Tree, tpt: Tree = TypeTree(), inlineable: Boolean)(implicit ctx: Context) = {
-    var mods = synthetic
+    var mods = synthetic | Artifact
     if (inlineable) mods |= Inline
     Block(
       DefDef(nme.ANON_FUN, Nil, params :: Nil, tpt, body).withMods(mods),