Refactor variance diagnostics to work with more types

Instead of special-casing mutable pointers/references, we
now support general generic types (currently, we handle
`ty::Ref`, `ty::RawPtr`, and `ty::Adt`)

When a `ty::Adt` is involved, we show an additional note
explaining which of the type's generic parameters is
invariant (e.g. the `T` in `Cell<T>`). Currently, we don't
explain *why* a particular generic parameter ends up becoming
invariant. In the general case, this could require printing
a long 'backtrace' of types, so doing this would be
more suitable for a follow-up PR.

We still only handle the case where our variance switches
to `ty::Invariant`.

compiler/rustc_infer/src/infer/combine.rs

@@ -572,8 +572,9 @@ impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
             // (e.g., #41849).
             relate::relate_substs(self, None, a_subst, b_subst)
         } else {
-            let opt_variances = self.tcx().variances_of(item_def_id);
-            relate::relate_substs(self, Some(&opt_variances), a_subst, b_subst)
+            let tcx = self.tcx();
+            let opt_variances = tcx.variances_of(item_def_id);
+            relate::relate_substs(self, Some((item_def_id, &opt_variances)), a_subst, b_subst)
         }
     }