Avoid too eager transform of $outer for lhs & accessor rhs

[szymon-rd]

Fix #18927

The transformer in mapOuter in Constructors was transforming trees it should not:

   override def transform(tree: Tree)(using Context) = tree match {
        [....]
        case tree: RefTree if tree.symbol.is(ParamAccessor) && tree.symbol.name == nme.OUTER =>
          ref(outerParam)
       [...]

There were two problems:

• This case transformed LHS of $outer assignments in constructors. So, instead of setting the $outer field in the current class with the constructor, it was replaced with the $outer of the outer class. That resulted in not assigning any value to the $outer in inner class, and double assignment to the val in outer class.
• LHS of the accessor def was also transformed, so it was evaluated to the $outer of the outer class.

This only happened when the nested class is created in the secondary constructor, as the primary constructor is not transformed (only the template body)

[odersky]

Nice catch!