When adding the #[ReturnTypeWillChange] attribute in namespaced
code, you also need to use ReturnTypeWillChange, otherwise it
will be silently ignored and may result in confusion. Change the
deprecation message to suggest #[\ReturnTypeWillChange], which
will always work.
Closes GH-7454.
Now that inheritance can throw deprecations again, these may be
converted to exception by a custom error handler. In this case
we need to convert the exception to a fatal error, as inheritance
cannot safely throw in the general case.