Auto merge of #35162 - canndrew:bang_type_coerced, r=nikomatsakis

Auto merge of #35162 - canndrew:bang_type_coerced, r=nikomatsakis

Implement the `!` type

This implements the never type (`!`) and hides it behind the feature gate `#[feature(never_type)]`. With the feature gate off, things should build as normal (although some error messages may be different). With the gate on, `!` is usable as a type and diverging type variables (ie. types that are unconstrained by anything in the code) will default to `!` instead of `()`.