E0122 "trait bounds are not (yet) enforced in type definitions" not triggered by `where` clauses

[cjsut]

As this playpen shows, diagnostic E0122 does not catch trait bounds introduced via where clauses.

Specifically,

type X<T: std::ops::Add> = T;

triggers a warning, but

type Y<T> where T: std::ops::Add = T;

does not.

[brson]

Looks like something that should be locked down.

[circuitfox]

I think I'll take a shot at this.