Rollup merge of #99011 - oli-obk:UnsoundCell, r=eddyb

`UnsafeCell` blocks niches inside its nested type from being available outside

fixes #87341

This implements the plan by `@eddyb` in https://github.com/rust-lang/rust/issues/87341#issuecomment-886083646

Somewhat related PR (not strictly necessary, but that cleanup made this PR simpler): #94527

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -96,13 +96,13 @@ impl Qualif for HasMutInterior {
     }
 
     fn in_adt_inherently<'tcx>(
-        cx: &ConstCx<'_, 'tcx>,
+        _cx: &ConstCx<'_, 'tcx>,
         adt: AdtDef<'tcx>,
         _: SubstsRef<'tcx>,
     ) -> bool {
         // Exactly one type, `UnsafeCell`, has the `HasMutInterior` qualif inherently.
         // It arises structurally for all other types.
-        Some(adt.did()) == cx.tcx.lang_items().unsafe_cell_type()
+        adt.is_unsafe_cell()
     }
 }