Callsites could be marked as builtin while calling nobuiltin
functions. This can lead to problems, if local optimizations apply
transformations based on the semantics of the builtin, but then IPO
treats the function as nobuiltin and applies a transform that breaks
builtin semantics (assumed earlier).
To avoid this, mark such functions as maybey-derefined, to avoid IPO
transforms on them that may break assumptions of earlier calls.