Rollup merge of #108923 - spastorino:new-rpitit-9, r=compiler-errors

Make fns from other crates with RPITIT work for -Zlower-impl-trait-in-trait-to-assoc-ty

Only the last two commits are meaningful.

r? `@compiler-errors`

compiler/rustc_hir/src/definitions.rs

@@ -404,8 +404,12 @@ impl DefPathData {
         match *self {
             TypeNs(name) | ValueNs(name) | MacroNs(name) | LifetimeNs(name) => Some(name),
 
+            // We use this name when collecting `ModChild`s.
+            // FIXME this could probably be removed with some refactoring to the name resolver.
+            ImplTraitAssocTy => Some(kw::Empty),
+
             Impl | ForeignMod | CrateRoot | Use | GlobalAsm | ClosureExpr | Ctor | AnonConst
-            | ImplTrait | ImplTraitAssocTy => None,
+            | ImplTrait => None,
         }
     }