Make ABI declaration a special case of trait declaration in AST

Since we allow supertraits for ABIs and will soon allow superabis for ABIs, traits and ABIs are pretty much the same thing and hence could be represented more uniformly. I propose `ty::TyDeclaration::TraitDeclaration` and `ty::TyDeclaration::AbiDeclaration` should be the same enum variant with a field that lets us tell the difference. This should simplify the implementation of #4049.

Start by reading the `ty::TyDeclaration` enum and the implementation of #4049, then trace how trait and ABI declarations are handled in the AST. The work is done when both declarations use one variant with a field distinguishing them and the related #4049 behavior remains correct.

Written by the indexing model from the issue text.