Rollup merge of #120498 - compiler-errors:type-flags, r=lcnr

Uplift `TypeVisitableExt` into `rustc_type_ir`

This uplifts `TypeVisitableExt` into `rustc_type_ir` so it can be used in an interner-agnostic way. It also moves some `TypeSuperVisitable` bounds onto `Interner` since we don't expect to support libraries that have types which aren't foldable by default.

This restores a couple of asserts in the canonicalizer code we uplifted, and also makes it so that we can use type-flags-based helpers in the solver code, which I'm interested in uplifting.

r? lcnr

compiler/rustc_middle/src/ty/mod.rs

@@ -504,6 +504,16 @@ impl<'tcx> IntoKind for Ty<'tcx> {
     }
 }
 
+impl<'tcx> rustc_type_ir::visit::Flags for Ty<'tcx> {
+    fn flags(&self) -> TypeFlags {
+        self.0.flags
+    }
+
+    fn outer_exclusive_binder(&self) -> DebruijnIndex {
+        self.0.outer_exclusive_binder
+    }
+}
+
 impl EarlyParamRegion {
     /// Does this early bound region have a name? Early bound regions normally
     /// always have names except when using anonymous lifetimes (`'_`).