This replaces regclassin with custom lookups of the namespace and relname, but misses some of the complexities that regclassin is handling. For instance, it calls RangeVarGetRelid(), which calls LookupExplicitNamespace(), which handles temp tables and InvokeNamespaceSearchHook().
At first it looked like a bit too much code to copy, but regclassin() passes NoLock, which means we basically just have to call LookupExplicitNamespace().
To be clear, LookupExplicitNamespace() can call aclcheck_error(), which is something we cannot presently step-down into a WARNING, so an aclcheck failure inside a restore/upgrade would fail the upgrade. I want to make sure we can live with that because it might be hard to explain what's an error we can nerf and what isn't.
