Refactor inner attribute parsing

[dtolnay]

Appending inner attrs onto an existing attributes vector, instead of into a separate vector which the caller is responsible for combining together, is more amenable for an upcoming refactor -- it's more convenient to pass &mut attrs as an argument to various helpers, rather than trying to pass back inner attrs as a tuple element in return position, which is always clumsy.

// easy
let thing = parse_thing(input, x, y, attrs)?;

// clumsy
let (thing, inner_attrs) = parse_thing(input, x, y)?;
attrs.extend(inner_attrs);

The rustc parser follows the same strategy, such as in https://github.com/rust-lang/rust/blob/b849326744a8eec939e592f0ab13bff85cc865d3/compiler/rustc_parse/src/parser/item.rs#L33-L48.