Add attrs inside WherePredicate::Lifetime and WherePredicate::Type

[dtolnay]

• RFC: Allow cfg-attributes in where clauses rust-lang/rfcs#3399
• Tracking issue: Tracking Issue for cfg_attribute_in_where rust-lang/rust#115590

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