This is for adding a guard on a non-immortal object, since we need
to be safe against that object getting deallocated and a different object
is allocated in its spot.

We had support for this already, but it leaked memory.  The biggest was
that we never freed our runtimeICs, so if those ended up getting any GC
references in them, then we would leak memory.  So I started freeing those,
but then that exposed the issue that the ICInvalidators expect that their
dependent ICs never get freed.  So I added back a mapping from ICSlotInfo->
ICInvalidators that reference them.