WF of a type variable unconditionally evaluates to `Yes`

In [`compute_well_formed_goal`](https://github.com/rust-lang/rust/blob/54d6738a8df74382c439e1dfb9ce4e2382d7742e/compiler/rustc_trait_selection/src/solve/mod.rs#L147), when given a goal `WF(?0)`,  it adds  the same input goal as a recursive goal, which is then evaluated to `Yes` because of WF goals being coinductive.

TODO: write a test (it seems hard to exploit).

