This change remains separate from the addition of adt.rs, even though
it's necessary for compatibility with pre-trans::adt representation,
to serve as an example of a change to the representation logic.
Later changes on this branch adapt the rest of rustc::middle::trans
to use this module instead of scattered hard-coded knowledge of
representations; a few of them also have improvements or cleanup for
adt.rs (and many added comments) that weren't drastic enough to justify
changing history to move them into this commit.
