Rollup merge of #136164 - celinval:chores-fnkind, r=oli-obk

Refactor FnKind variant to hold &Fn

Pulling the change suggested in #128045 to reduce the impact of changing `Fn` item.

r? `@oli-obk`

compiler/rustc_lint/src/builtin.rs

@@ -330,10 +330,12 @@ impl EarlyLintPass for UnsafeCode {
         if let FnKind::Fn(
             ctxt,
             _,
-            ast::FnSig { header: ast::FnHeader { safety: ast::Safety::Unsafe(_), .. }, .. },
             _,
-            _,
-            body,
+            ast::Fn {
+                sig: ast::FnSig { header: ast::FnHeader { safety: ast::Safety::Unsafe(_), .. }, .. },
+                body,
+                ..
+            },
         ) = fk
         {
             let decorator = match ctxt {