Use mangled suffix for synthetic setters

Author: odersky

compiler/src/dotty/tools/dotc/core/NameKinds.scala

@@ -367,7 +367,7 @@ object NameKinds {
   val ModuleClassName: SuffixNameKind = new SuffixNameKind(OBJECTCLASS, "$", optInfoString = "ModuleClass")
   val ImplMethName: SuffixNameKind = new SuffixNameKind(IMPLMETH, "$")
   val AdaptedClosureName: SuffixNameKind = new SuffixNameKind(ADAPTEDCLOSURE, "$adapted") { override def definesNewName = true }
-  val SyntheticSetterName: SuffixNameKind = new SuffixNameKind(SETTER, "_=")
+  val SyntheticSetterName: SuffixNameKind = new SuffixNameKind(SETTER, "_$eq")
 
   /** A name together with a signature. Used in Tasty trees. */
   object SignedName extends NameKind(SIGNED) {