Remove hook calling via TyCtxtAt.

All hooks receive a `TyCtxtAt` argument.

Currently hooks can be called through `TyCtxtAt` or `TyCtxt`. In the
latter case, a `TyCtxtAt` is constructed with a dummy span and passed to
the hook.

However, in practice hooks are never called through `TyCtxtAt`, and
always receive a dummy span. (I confirmed this via code inspection, and
double-checked it by temporarily making the `TyCtxtAt` code path panic
and running all the tests.)

This commit removes all the `TyCtxtAt` machinery for hooks. All hooks
now receive `TyCtxt` instead of `TyCtxtAt`. There are two existing hooks
that use `TyCtxtAt::span`: `const_caller_location_provider` and
`try_destructure_mir_constant_for_user_output`. For both hooks the span
is always a dummy span, probably unintentionally. This dummy span use is
now explicit. If a non-dummy span is needed for these two hooks it would
be easy to add it as an extra argument because hooks are less
constrained than queries.

compiler/rustc_incremental/src/lib.rs

@@ -24,7 +24,7 @@ use rustc_middle::util::Providers;
 #[allow(missing_docs)]
 pub fn provide(providers: &mut Providers) {
     providers.hooks.save_dep_graph =
-        |tcx| tcx.sess.time("serialize_dep_graph", || persist::save_dep_graph(tcx.tcx));
+        |tcx| tcx.sess.time("serialize_dep_graph", || persist::save_dep_graph(tcx));
 }
 
 rustc_fluent_macro::fluent_messages! { "../messages.ftl" }