Rollup merge of #112703 - aliemjay:next-solver-root-var, r=compiler-errors

[-Ztrait-solver=next, mir-typeck] instantiate hidden types in the root universe

Fixes an ICE in the test `member-constraints-in-root-universe`.

Main motivation is to make #112691 pass under the new solver.

r? ``@compiler-errors``

compiler/rustc_infer/src/infer/mod.rs

@@ -1474,6 +1474,7 @@ impl<'tcx> InferCtxt<'tcx> {
     /// universes. Updates `self.universe` to that new universe.
     pub fn create_next_universe(&self) -> ty::UniverseIndex {
         let u = self.universe.get().next_universe();
+        debug!("create_next_universe {u:?}");
         self.universe.set(u);
         u
     }