Reorder fields in hir::ItemKind variants.

Specifically `TyAlias`, `Enum`, `Struct`, `Union`. So the fields match
the textual order in the source code.

The interesting part of the change is in
`compiler/rustc_hir/src/hir.rs`. The rest is extremely mechanical
refactoring.

compiler/rustc_lint/src/default_could_be_derived.rs

@@ -95,8 +95,8 @@ impl<'tcx> LateLintPass<'tcx> for DefaultCouldBeDerived {
                 kind:
                     hir::ItemKind::Struct(
                         _,
-                        hir::VariantData::Struct { fields, recovered: _ },
                         _generics,
+                        hir::VariantData::Struct { fields, recovered: _ },
                     ),
                 ..
             })) => fields.iter().map(|f| (f.ident.name, f)).collect::<FxHashMap<_, _>>(),