rustc: Migrate `__register_diagnostic` and friends to stable proc macros

[petrochenkov]

A few macros used internally in rustc for working with error codes still use legacy proc macros (aka syntactic plugins) making rustc one of the last (the last?) noticeable users of the feature in the ecosystem.

They need to be moved to librustc_macros and re-implemented using the stable proc macro API.
Currently they are registered in librustc_interface/passes.rs - __diagnostic_used, __register_diagnostic and __build_diagnostic_array.

(If they can be rewritten as declarative macros, that would be even better.)

[petrochenkov]

cc @GuillaumeGomez @estebank
(I'm not sure who is responsible for the error index now.)

[Mark-Simulacrum]

Assigning to self as I've recently done some work here so should be able to knock this out fairly easily.