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.

src/librustdoc/passes/calculate_doc_coverage.rs

@@ -241,7 +241,7 @@ impl DocVisitor<'_> for CoverageCalculator<'_, '_> {
                                 data: hir::VariantData::Tuple(_, _, _),
                                 ..
                             }) | hir::Node::Item(hir::Item {
-                                kind: hir::ItemKind::Struct(_, hir::VariantData::Tuple(_, _, _), _),
+                                kind: hir::ItemKind::Struct(_, _, hir::VariantData::Tuple(_, _, _)),
                                 ..
                             })
                         )