Parse attributes on where-predicates

[dtolnay]

• RFC: https://github.com/rust-lang/rfcs/pull/3399
• Tracking issue: https://github.com/rust-lang/rust/issues/115590

impl<T> SomeTrait<T> for Thing
where
    #[cfg(something_a)] T: SomeRequirementA,
    #[cfg(something_b)] T: SomeRequirementB,
{}