HRTB test cases from rust-lang/rust

[nikomatsakis]

Per this conversation, we should model:

• Functions, closures, and HRTB-trait-objects can implement traits such that validity of associated types is never checked. rust#84533
• HRTB on subtrait unsoundly provides HTRB on supertrait with weaker implied bounds rust#84591
• Unsoundness due to where clauses not checked for well-formedness rust#98117