let generate_field_attrs_code create FieldInfo

this simplifies the code inside the `structure.each` closure argument
and allows to remove the `vis` field from `FieldInfo`.

compiler/rustc_macros/src/diagnostics/subdiagnostic.rs

@@ -303,7 +303,6 @@ impl<'a> SessionSubdiagnosticDeriveBuilder<'a> {
 
         let inner_ty = FieldInnerTy::from_type(&ast.ty);
         let info = FieldInfo {
-            vis: &ast.vis,
             binding: binding,
             ty: inner_ty.inner_type().unwrap_or(&ast.ty),
             span: &ast.span(),