Properly parse the ACC_SYNTHETIC bytecode flag

ACC_SYNTHETIC is used to mark definitions that are pure implementation
artifacts and should not affect typechecking, this is normally
represented using the Artifact flag in Scala, but for some reason we
were missing it compared to Scala 2:
https://github.com/scala/scala/blob/8f20a7d84924939698717d26b3e0e5a5c0da829c/src/reflect/scala/reflect/internal/ClassfileConstants.scala#L352

Author: smarter

compiler/src/dotty/tools/dotc/core/classfile/ClassfileConstants.scala

@@ -341,7 +341,7 @@ object ClassfileConstants {
       case JAVA_ACC_PRIVATE    => Private
       case JAVA_ACC_PROTECTED  => Protected
       case JAVA_ACC_FINAL      => Final
-      case JAVA_ACC_SYNTHETIC  => Synthetic
+      case JAVA_ACC_SYNTHETIC  => SyntheticArtifact
       case JAVA_ACC_STATIC     => JavaStatic
       case JAVA_ACC_ENUM       => Enum
       case JAVA_ACC_ABSTRACT   => if (isClass) Abstract else Deferred