Auto merge of #51895 - nikomatsakis:move-self-trait-predicate-to-items, r=scalexm

Move self trait predicate to items

This is a "reimagination" of @tmandry's PR #50183. The main effect is described in this comment from one of the commits:

---

Before we had the following results for `predicates_of`:

```rust
trait Foo { // predicates_of: Self: Foo
  fn bar(); // predicates_of: Self: Foo (inherited from trait)
}
```

Now we have removed the `Self: Foo` from the trait. However, we still
add it to the trait ITEM. This is because when people do things like
`<T as Foo>::bar()`, they still need to prove that `T: Foo`, and
having it in the `predicates_of` seems to be the cleanest way to
ensure that happens right now (otherwise, we'd need special case code
in various places):

```rust
trait Foo { // predicates_of: []
  fn bar(); // predicates_of: Self: Foo
}
```

However, we sometimes want to get the list of *just* the predicates
truly defined on a trait item (e.g., for chalk, but also for a few
other bits of code). For that, we define `predicates_defined_on`,
which does not contain the `Self: Foo` predicate yet, and we plumb
that through metadata and so forth.

---

I'm assigning @eddyb as the main reviewer, but I thought I might delegate to scalexm for this one in any case. I also want to post an alternative that I'll leave in the comments; it occurred to me as I was writing. =)

r? @eddyb
cc @scalexm @tmandry @leodasvacas

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