Remove return type sized check hack from hir typeck
Remove a bunch of special-cased suggestions when someone returns `-> dyn Trait` that checks for type equality, etc.
This was a pretty complex piece of code that also relied on a hack in hir typeck (see changes to `compiler/rustc_hir_typeck/src/check.rs`), and I'm not convinced that it's necessary to maintain, when all we really need to tell the user is that they should return `-> impl Trait` or `-> Box<dyn Trait>`, depending on their specific use-case.
This is necessary because we may need to move the "return type is sized" check from hir typeck to wfcheck, which does not have access to typeck results. This is a prerequisite for that, and I'm fairly confident that the diagnostics "regressions" here are not a big deal.
Dylan DPC
4b26b80dd5