Rollup merge of #160914 - ada4a:GenericTypeVisitable-bounds, r=JonathanBrouwer

Derive `GenericTypeVisitable` for `RegionConstraint` _correctly_

The derive added in https://github.com/rust-lang/rust/pull/160164 was incorrect -- it resulted in an overflow during trait solving. This is because `#[derive(GenericTypeVisitable)]` automatically adds a `: GenericTypeVisitable` bound to every field of a type -- in this case, `Box<[RegionConstraint<I>]>: GenericTypeVisitable<V>`.

To fix this, I added a `#[generic_type_visitable(bounds(..))]` attribute to the derive macro, which allows overriding the added bounds.

I also made the derive macro no longer a no-op in rustc, so that errors like this can be caught on r-l/r CI in the future.

Best reviewed commit-by-commit.

cc @ChayimFriedman2