scavenge_one failures on BCO objects

[chessai .]

Hi devs,

A coworker is experiencing sporadic failures when reloading our
project in GHCi, with

```
ghc: internal error: scavenge_one: strange object 23
    (GHC version 8.6.5 for x86_64_unknown_linux)
    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
cabal: repl failed for mercury-web-backend-0.0.0. The build process terminated
with exit code -6
```

upon a quick inspection it seems that scavenge_one inspects the type
of the object, but has no case for BCO (which is 23). I'm guessing
there's something weird going on here since this doesn't happen
deterministically.

I don't have a good intuition for how this scavenging should work for
BCO's, or if it should even be possible for BCO's to appear here (i.e.
this is a symptom of some other bug).

Any thoughts?

Thanks