Auto merge of rust-lang#82964 - Nicholas-Baron:shorten_middle_ty, r=jackh726

Shorten `rustc_middle::ty::mod`

Related to rust-lang#60302.

This PR moves all `Adt*`, `Assoc*`, `Generic*`, and `UpVar*` types to separate files.
This, alongside some `use` reordering, puts `mod.rs` at ~2,200 lines, thus removing the `// ignore-tidy-filelength`.

The particular groups were chosen as they had 4 or more "substantive" members.

Author: bors