libsyntax/librustc_front: Some visitor methods don't have access to span information

[petrochenkov]

Namely:

visit_struct_def
visit_enum_def
visit_generics
visit_arm (can use span of inner Expr, but it's bad)

+ maybe

visit_trait_ref (can use span of inner Path, acceptable)
visit_ty_param_bound (can use span of inner Lifetime or PolyTraitRef, acceptable)
visit_lifetime_def (can use span of inner Lifetime, acceptable)

So, if I want to visit, for example, a StructDef and report an error, I can't do it and have to visit all its parents instead.