Rollup merge of #114267 - compiler-errors:rpitit-opaque-bounds, r=spastorino

Map RPITIT's opaque type bounds back from projections to opaques

An RPITIT in a program's AST is eventually translated into both a projection GAT and an opaque. The opaque is used for default trait methods, like:

```
trait Foo {
  fn bar() -> impl Sized { 0i32 }
}
```

The item bounds for both the projection and opaque are identical, and both have a *projection* self ty. This is mostly okay, since we can normalize this projection within the default trait method body to the opaque, but it does two things:
1. it leads to bugs in places where we don't normalize item bounds, like `deduce_future_output_from_obligations`
2. it leads to extra match arms that are both suspicious looking and also easy to miss

This PR maps the opaque type bounds of the RPITIT's *opaque* back to the opaque's self type to avoid this quirk. Then we can fix the UI test for #108304 (1.) and also remove a bunch of match arms (2.).

Fixes #108304

r? `@spastorino`

compiler/rustc_middle/src/ty/mod.rs

@@ -2625,19 +2625,6 @@ impl<'tcx> TyCtxt<'tcx> {
         matches!(self.trait_of_item(def_id), Some(trait_id) if self.has_attr(trait_id, sym::const_trait))
     }
 
-    pub fn impl_trait_in_trait_parent_fn(self, mut def_id: DefId) -> DefId {
-        match self.opt_rpitit_info(def_id) {
-            Some(ImplTraitInTraitData::Trait { fn_def_id, .. })
-            | Some(ImplTraitInTraitData::Impl { fn_def_id, .. }) => fn_def_id,
-            None => {
-                while let def_kind = self.def_kind(def_id) && def_kind != DefKind::AssocFn {
-                    def_id = self.parent(def_id);
-                }
-                def_id
-            }
-        }
-    }
-
     /// Returns the `DefId` of the item within which the `impl Trait` is declared.
     /// For type-alias-impl-trait this is the `type` alias.
     /// For impl-trait-in-assoc-type this is the assoc type.