Auto merge of #34193 - petrochenkov:privalias, r=nikomatsakis

Auto merge of #34193 - petrochenkov:privalias, r=nikomatsakis

privacy: Substitute type aliases in private-in-public checker

Closes https://github.com/rust-lang/rust/issues/30503
Closes https://github.com/rust-lang/rust/issues/34293

Everyone in the issue discussion seemed to be in favor, @huonw also spoke about this [here](https://www.reddit.com/r/rust/comments/3xldr9/surfaces_and_signatures_component_privacy_versus/cy615wq), but the issue haven't got any movement.
I think it's reasonable to do this before turning `private_in_public` warnings into errors.

r? @nikomatsakis