MPS: assertion failed: header_type (h) != IGC_OBJ_FWD

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

I'm seeing this assertion sometimes in an Emacs built with
--enable-checking=igc_debug,igc_check_fwd,w here sometines means it can
take days of using/running Emacs, or it can take a couple of hours.
This is macOS 14, arm64. I'm linking with -lmps-debug.

The assertion means that we likely have a reference somewhere that
isn't traced. Because it isn't traced, the reference isn't changed to
point to the new location when the object being references is copied
to a new address in memory. Instead, it points to kind of a tombstone
that is left behind when the object is moved.

Alas, I haven't been able to debug this. One problem is that I
can't reproduce it easily, the other is that is is either not
happening or happening much less often when building with -O0, and
without -O0 I can't see much here.

This is just to let people know of the problem. If you find a recipe
how to reproduce this, please let me know. Or better yet, debug it :-).