Auto merge of #36066 - jseyfried:rollup, r=Manishearth

Auto merge of #36066 - jseyfried:rollup, r=Manishearth

Batch up libsyntax breaking changes

Batch of the following syntax-[breaking-change] changes:
 - #35591: Add a field `span: Span` to `ast::Generics`.
 - #35618: Remove variant `Mod` of `ast::PathListItemKind` and refactor the remaining variant `ast::PathListKind::Ident` to a struct `ast::PathListKind_`.
 - #35480: Change uses of `Constness` in the AST to `Spanned<Constness>`.
  - c.f. `MethodSig`, `ItemKind`
 - #35728: Refactor `cx.pat_enum()` into `cx.pat_tuple_struct()` and `cx.pat_path()`.
 - #35850: Generalize the elements of lists in attributes from `MetaItem` to a new type `NestedMetaItem` that can represent a `MetaItem` or a literal.
 - #35917: Remove traits `AttrMetaMethods`, `AttributeMethods`, and `AttrNestedMetaItemMethods`.
  - Besides removing imports of these traits, this won't cause fallout.
 - Add a variant `Union` to `ItemKind` to future proof for `union` (c.f. #36016).
 - Remove inherent methods `attrs` and `fold_attrs` of `Annotatable`.
  - Use methods `attrs` and `map_attrs` of `HasAttrs` instead.

r? @Manishearth