Fix setter name for Scala 2 traits

Solution: only use simple name for fields

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_=]

However, the field created by the getter will be:

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

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

After the change, the following field is generated instead:

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

Now, the setter will be able to find the field via `sym.field`.
This fix avoids generate a phantom symbol without corresponding trees.

Author: liufengyun

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

@@ -270,7 +270,7 @@ object NameOps {
           original.fieldName
         }
         else getterName.fieldName
-      else FieldName(name)
+      else FieldName(name.toSimpleName)
 
     def stripScala2LocalSuffix: TermName =
       if (name.isScala2LocalSuffix) name.asSimpleName.dropRight(1) else name