Auto merge of #123332 - Nadrieril:testkind-never, r=matthewjasper

never patterns: lower never patterns to `Unreachable` in MIR

This lowers a `!` pattern to "goto Unreachable". Ideally I'd like to read from the place to make it clear that the UB is coming from an invalid value, but that's tricky so I'm leaving it for later.

r? `@compiler-errors` how do you feel about a lil bit of MIR lowering

Author: bors