[fix #18977] disallow change branch of an object variant in ARC again

[ringabout]

fix #18977

todo

• make {.cast(uncheckedAssign).} work

[Araq]

I don't understand this.. You merely updated some tests?

[ringabout]

Yeah, I haven't figured out a solution.

[planetis-m]

fyi, the discriminator destructor is not enough to ensure use after free bugs from happening, I have a ton in my project. One needs to emit this code instead:

if a.kind != tmpkind:
  case a.kind
  of X: # only destroy X fields (no reset)
  of Y: # only destroy Y fields
  a.kind = tmpKind
  case a.kind
  of X: # reset X fields
  of Y: # reset Y fields