Fix Erasure of uniqueRefDenotations with different underlying type.

The bug happened for `new Tuple2[Int, Int](1, 2)._1`, if the last
`._1` selection is done by symbol. This will create a
uniqueRefDenotation with underlying type Int.

This denotation has to be erased in conformance with underlying type,
not with the observed type before erasure, as otherwise you'd think
that `_1` on topple returns Int.

Author: DarkDimius

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

@@ -440,7 +440,7 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
         // but potentially re-introduced by ResolveSuper, when we add
         // forwarders to mixin methods.
         // See doc comment for ElimByName for speculation how we could improve this.
-      else MethodType(Nil, Nil, eraseResult(rt))
+      else MethodType(Nil, Nil, eraseResult(sym.info.finalResultType))
     case tp: PolyType =>
       eraseResult(tp.resultType) match {
         case rt: MethodType => rt