MINOR: Comment ty::TraitStore and ty::vstore
Add trait object field types to back/abi.rs, and use them

I've added trt_field_vtable, trt_field_box, and trt_field_tydesc, and
inserted them in place of the "magic numbers" used to access trait
object fields through GEPi().
WIP: Add AutoBorrowTraitObj AutoRefKind, and start implementing relevant adjustments

This partially implements (1) and (3) from my IRC conversation with
Niko:

 (1) add a new kind of adjustment for objects
 (3) modify the method code to borrow in search_for_autosliced_method()

I also added FIXMEs to things I wasn't sure about. Remove them if the
behaviour is correct, or fixed in the future.

cc #5762
WIP: Implement auto-borrow adjustment for trait objects in trans/expr.rs

cc #5762 (incomplete fix)