Improvements to attribute suggestions Changes in commit 1: - Add `AcceptContext::suggestions`, which retrieves the suggestions for the currently parsing attribute - This happens to fix a bug in the way `#[macro_export`]. Closes https://github.com/rust-lang/rust/pull/147987 Changes in commit 2: - Add a check to the suggestions function so the suggestions for attributes in cfg_attr are nicer. Fixes https://github.com/rust-lang/rust/issues/147693 This is also part (but not all) of the changes needed to unblock https://github.com/rust-lang/rust/pull/147945 r? `@jdonszelmann`