Rollup merge of #125750 - compiler-errors:expect, r=lcnr

Align `Term` methods with `GenericArg` methods, add `Term::expect_*`

* `Term::ty` -> `Term::as_type`.
* `Term::ct` -> `Term::as_const`.
* Adds `Term::expect_type` and `Term::expect_const`, and uses them in favor of `.ty().unwrap()`, etc.

I could also shorten these to `as_ty` and then do `GenericArg::as_ty` as well, but I do think the `as_` is important to signal that this is a conversion method, and not a getter, like `Const::ty` is.

r? types

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -946,7 +946,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 // since we don't actually use them.
                 &mut vec![],
             )
-            .ty()
+            .as_type()
             .unwrap();
 
             if let ty::Dynamic(data, ..) = ty.kind() { data.principal() } else { None }