Add note to non-exhaustive match on reference to empty

Rust prints "type `&A` is non-empty" even is A is empty.
This is the intended behavior, but can be confusing.
This commit adds a note to non-exhaustive pattern errors if they are a
reference to something uninhabited.

I did not add tests to check that the note is not shown for
non-references or inhabited references, because this is already done
in other tests.

Maybe the added test is superfluous, because
`always-inhabited-union-ref` already checks for this case.

This does not handle &&Void or &&&void etc. I could add those as special
cases as well and ignore people who need quadruple
references.

Fixes #78123

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -503,6 +503,11 @@ fn non_exhaustive_match<'p, 'tcx>(
             ));
         }
     }
+    if let ty::Ref(_, sub_ty, _) = scrut_ty.kind() {
+        if cx.tcx.is_ty_uninhabited_from(cx.module, sub_ty, cx.param_env) {
+            err.note("references are always considered inhabited");
+        }
+    }
     err.emit();
 }