[backport] fixes #23690; SIGSEGV with object variants and RTTI (#23703)

fixes #23690

```nim
dest.`:state` = src.`:state`
var :tmp_553651276 = dest.e1.a
`=wasMoved`(dest.e1.a)
dest.e1.a.kind = src.e1.a.kind
case dest.e1.a.kind
of 0:
  dest.e1.a.a = src.e1.a.a
of 1:
  `=copy`(dest.e1.a.c, src.e1.a.c)
case :tmp_553651276.kind
of 0:
of 1:
  `=destroy`(:tmp_553651276.c)
```
`dest.e1.a.kind = src.e1.a.kind` changes the discrimant but it fails to
clear the memory of `dest.e1.a`. Before using hooks for copying, we need
to clear the dest, e.g. `=wasMoved(dest.e1.a.c)`.

```nim
dest.`:state` = src.`:state`
var :tmp_553651276 = dest.e1.a
`=wasMoved`(dest.e1.a)
dest.e1.a.kind = src.e1.a.kind
case dest.e1.a.kind
of 0:
  `=wasMoved`(dest.e1.a.a)
  dest.e1.a.a = src.e1.a.a
  `=wasMoved`(dest.e1.a.b)
of 1:
  `=wasMoved`(dest.e1.a.c)
  `=copy`(dest.e1.a.c, src.e1.a.c)
case :tmp_553651276.kind
of 0:
of 1:
  `=destroy`(:tmp_553651276.c)
```

0 files changed, 0 insertions, 0 deletions