The existence of a shared_external function in itself is not
an error; it just means we deserialized a witness table or
vtable but did not need to deserialize a thunk.
However, a direct reference to such a function is an error,
because we should have deserialized the body in that case.
This fixes a crasher, but the SIL crashers are kind of silly
because the SIL parser does not try at all not to crash on
invalid input.
