Rollup merge of #34339 - jseyfried:thin_vec, r=petrochenkov,Manishearth

Generalize and abstract `ThinAttributes` to `ThinVec<Attribute>`.

src/libsyntax_ext/asm.rs

@@ -261,6 +261,6 @@ pub fn expand_asm<'cx>(cx: &'cx mut ExtCtxt, sp: Span, tts: &[tokenstream::Token
             expn_id: expn_id,
         }),
         span: sp,
-        attrs: None,
+        attrs: ast::ThinVec::new(),
     }))
 }