Problem: methods can be removed or renamed, and their overrides can be missed in the meantime, generating dead code and possibly losing features / adding bugs which can be hard to track.

On registry load, looks for `super` in methods, tries to find out what they're looking for in a superclass and checks if that thing exists. Has some impact on registry loading (on my machine, with 500 modules loaded the check costs 1.5s to ~10s, it also shouldn't cost that much on re-check e.g. module installations as the classes are "cached").

TODO: probably remove the jump handler for 3.5? I don't think we support that in master.
TODO: check the 15 methods it's currently finding to see what's to fix e.g. remove outdated method or rename an override of a renamed method

From a request of @fmdl in #53450, this solution is more CPU-expensive than an explicit `@overrides` but generates far less code churn to produce results: methods don't need to be annotated to be checked, meaning we can't forget annotating methods (unless we make the annotation a requirement à la C# I guess, but that would be *a lot* of code churn).