Auto merge of #49711 - ibabushkin:auto_trait_refactor, r=nikomatsakis

Refactor auto trait handling in librustdoc to be accessible from librustc.

These commits transfer some of the functionality introduced in #47833 to librustc with the intention of making the tools to work with auto traits accessible to third-party code, for example [rust-semverver](https://github.com/rust-lang-nursery/rust-semverver).

Some rough edges remain, and I'm certain some of the FIXMEs introduced will need some discussion, most notably the fairly ugly overall approach to pull out the core logic into librustc, which was previously fairly tightly coupled with various bits and bobs from librustdoc.

cc @Aaron1011