Rollup merge of #141675 - nnethercote:ItemKind-field-order, r=fee1-dead

Reorder `ast::ItemKind::{Struct,Enum,Union}` fields.

So they match the order of the parts in the source code, e.g.:
```
struct Foo<T, U> { t: T, u: U }
       <-><----> <------------>
       /   |       \
   ident generics  variant_data
```

r? `@fee1-dead`

Author: jhpratt

src/items.rs

@@ -1110,10 +1110,10 @@ impl<'a> StructParts<'a> {
 
     pub(crate) fn from_item(item: &'a ast::Item) -> Self {
         let (prefix, def, ident, generics) = match item.kind {
-            ast::ItemKind::Struct(ident, ref def, ref generics) => {
+            ast::ItemKind::Struct(ident, ref generics, ref def) => {
                 ("struct ", def, ident, generics)
             }
-            ast::ItemKind::Union(ident, ref def, ref generics) => ("union ", def, ident, generics),
+            ast::ItemKind::Union(ident, ref generics, ref def) => ("union ", def, ident, generics),
             _ => unreachable!(),
         };
         StructParts {

src/visitor.rs

@@ -521,7 +521,7 @@ impl<'b, 'a: 'b> FmtVisitor<'a> {
                 ast::ItemKind::Struct(..) | ast::ItemKind::Union(..) => {
                     self.visit_struct(&StructParts::from_item(item));
                 }
-                ast::ItemKind::Enum(ident, ref def, ref generics) => {
+                ast::ItemKind::Enum(ident, ref generics, ref def) => {
                     self.format_missing_with_indent(source!(self, item.span).lo());
                     self.visit_enum(ident, &item.vis, def, generics, item.span);
                     self.last_pos = source!(self, item.span).hi();