Get rid of HIR const checker As far as I can tell, the HIR const checker was implemented in https://github.com/rust-lang/rust/pull/66170 because we were not able to issue useful const error messages in the MIR const checker. This seems to have changed in the last 5 years, probably due to work like #90532. I've tweaked the diagnostics slightly and think the error messages have gotten *better* in fact. Thus I think the HIR const checker has reached the end of its usefulness, and we can retire it. cc `@RalfJung`