When we GC, we assert the invariant that all `externref`s we find on the stack
have a corresponding entry in the `VMExternRefActivationsTable`. However, we
also might be in code that is in the process of fixing up this invariant and
adding an entry to the table, but the table's bump chunk is full, and so we do a
GC and then add the entry into the table. This will ultimately maintain our
desired invariant, but there is a moment in time when we are doing the GC where
the invariant is relaxed which is okay because the reference will be in the
table before we return to Wasm or do anything else. This isn't a possible UAF,
in other words. To make it so that the assertion won't trip, we explicitly
insert the reference into the table *before* we GC, so that the invariant is not
relaxed across a possibly-GCing operation (even though it would be safe in this
particular case).
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