This removes another special case of Switch by replacing it with the more general SwitchInt. While
this is more clunky currently, there’s no reason we can’t make it nice (and efficient) to use.
Use it instead of a `panic` for inexhaustive matches and correct the
comment. I think we trust our match-generation algorithm enough to
generate these blocks, and not generating an `unreachable` means that
LLVM won't optimize `match void() {}` to an `unreachable`.
this introduces a DropAndReplace terminator as a fix to #30380. That terminator
is suppsoed to be translated by desugaring during drop elaboration, which is
not implemented in this commit, so this breaks `-Z orbit` temporarily.
The pass removes the unwind branch of each terminator, thus moving the responsibility of handling
the -Z no-landing-pads flag to a small self-contained pass… instead of polluting the translator.
