Perform many const checks in typeck Some smaller diagnostic changes, the biggest ones avoided by https://github.com/rust-lang/rust/pull/148641 We should be able to move various checks in mir const checking to using `span_bug!` instead of reporting an error, just like mir typeck does as a sanity check. I would like to start doing so separately though, as this PR is a big enough (in what effects it causes, pun intended). r? @fee1-dead