Fix setter name for Scala 2 traits

Before the change, given the following code

    class A extends App

The compiler will generate the following setter in the phase Mixin:

    scala[Qualified $ App][Qualified $_setter_$ scala$App$$initCode_=]

After the change, the following setter is generated instead:

    scala[Qualified $ App][Qualified $$ initCode_=]

The former will cause a problem in the phase Memoize in the line below:

    val field = sym.field.orElse(newField).asTerm

The problem is that, for the setter, `sym.field` will be unable to
find the field generated by the getter below, due to the name mismatch:

    scala[Qualified $ App][Qualified $$ initCode][Suffix $$local]

Consequently, the method `newField` will be called to create another
field symbol without corresponding trees.

This fix avoids generate a phantom symbol without corresponding trees.

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/AugmentScala2Traits.scala

@@ -48,10 +48,9 @@ class AugmentScala2Traits extends MiniPhase with IdentityDenotTransformer { this
 
   private def augmentScala2Trait(mixin: ClassSymbol)(implicit ctx: Context): Unit = {
     def traitSetter(getter: TermSymbol) =
+      val name = getter.ensureNotPrivate.name.asTermName.setterName
       getter.copy(
-        name = getter.ensureNotPrivate.name
-          .expandedName(getter.owner, TraitSetterName)
-          .asTermName.setterName,
+        name = name,
         flags = Method | Accessor,
         info = MethodType(getter.info.resultType :: Nil, defn.UnitType))