Deprecated attribute should be inherited

[aturon]

We've cycled through a few different inheritance strategies for stability attributes, but with the move to release channels we've lost inheritance for the deprecated attribute. This appears not to be an intentional choice, and there are strong ergonomic reasons to use inheritance here.

We should alter the lint's computation so that deprecation is inherited. Should be pretty easy to do.

[aturon]

I'm happy to mentor if someone wants to work on this.

[aturon]

cc @Manishearth

[ericye16]

I'd like to tackle this, if still available. I think my starting point is src/librustc/middle/stability.rs, is that correct?

[Gankra]

Whoops, sorry @ericye16, I started working on this earlier this morning and have a WIP PR up.

(I'm not super confident in the design, though)

[Gankra]

In particular this change seems very... noisy. It might be worth investigating coalescing deprecated API usage errors.