Rolls back certain changes that were introduced during typechecking of the reifee.
* Undoing macro expansions
* Replacing type trees with TypeTree(tpe)
* Reassembling CompoundTypeTrees into reifiable form
* Transforming Modifiers.annotations into Symbol.annotations
* Transforming Annotated annotations into AnnotatedType annotations
* Transforming Annotated(annot, expr) into Typed(expr, TypeTree(Annotated(annot, _))
* Non-idempotencies of the typechecker: https://issues.scala-lang.org/browse/SI-5464