Don't generate functions named 'nil'

[slburson]

Given something like this:

struct IValue {
    union Payload {
        union TriviallyCopyablePayload {
            TriviallyCopyablePayload() : as_int(0) {}
            long as_int;
            unsigned as_uint;
            double as_double;
            bool as_bool;
            struct {                   // <-- unnamed struct
                char type;
                char index;
            } as_device;
        } u;
        Payload(const Payload&) = delete;
        Payload(Payload&&) = delete;
        Payload& operator=(const Payload&) = delete;
        Payload& operator=(Payload&&) = delete;
    };
};

libclang generates an assignment operator for the unnamed struct. I'm not quite sure why it does that; as I recall, it didn't do it without the delete declarations, though I'm not sure all four are needed. Anyway, the outcome was that claw.iffi.cxx::symbolicate-function-name would return nil, which caused predictable breakage.