Add `#![rustc_never_type_mode = "..."]` crate-level attribute to allow experimenting
Demonstrating how different approaches with the never type work is somewhat hard when you can't actually provide a runnable example. Let's add features that change the fallback behavior.
This adds `#![rustc_never_type_mode = "no_fallback"]` and `#![rustc_never_type_mode = "fallback_to_never"]`, but I also plan to add others (in future PRs).
cc ``@traviscross``
r? ``@compiler-errors``
82d5b568b8