We used to manually purge `raise_exception_helper`, which has now been
split in `_abort` and `_unreachable`. We want to purge the latter but
keep the former. The latter is marked as `Exceptional`, so this commit
updates the logic to purge all functions with `Exceptional` tag.
Also, due to the inability of the backend to emit globals of `struct`
type, we ban `set_PlainMetaAddress`, which is not critical for
decompiled code.
A bug in revng allowed to use any TaggedFunctionKinds in place of any
other one. That bug has been fixed and the wrong usages in revng-c have
been updated.
Alessandro Di Federico