Auto merge of #154758 - WaffleLapkin:aliassss, r=lcnr

`ty::Alias` refactor



This PR changes the following alias-related types from this:

```rust
pub enum AliasTyKind {
    Projection,
    Inherent,
    Opaque,
    Free,
}

pub struct AliasTy<I: Interner> {
    pub args: I::GenericArgs,
    pub def_id: I::DefId,
}

pub enum TyKind<I: Interner> {
    ...
    Alias(AliasTyKind, AliasTy<I>),
}
```
Into this:

```rust
pub enum AliasTyKind<I: Interner> {
    Projection { def_id: I::DefId },
    Inherent { def_id: I::DefId },
    Opaque { def_id: I::DefId },
    Free { def_id: I::DefId },
}

pub struct AliasTy<I: Interner> {
    pub args: I::GenericArgs,
    pub kind: AliasTyKind<I>,
}

pub enum TyKind<I: Interner> {
    ...
    Alias(AliasTy<I>),
}
```

... and then does a thousand other changes to accommodate for this change everywhere.

This brings us closer to being able to have `AliasTyKind`s which don't require a `DefId` (and thus can be more easily created, etc). Although notably we depend on both `AliasTyKind -> DefId` and `DefId -> AliasTyKind` conversions in a bunch of places still.

r? lcnr

----

A lot of these changes were done either by search & replace (via `ast-grep`) or on auto pilot, so I'm not quite sure I didn't mess up somewhere, but at least tests pass...

Author: bors