Ignore -Wnested-anon-types when using Clang

[Psirus]

As it says in the title, this change will add the -Wnested-anon-types to the list of ignored warning flags under Clang. Solves #1204.

[jagerman]

In this case I think it might be cleaner to listen to the warning rather than ignoring it, i.e. by adding:

struct nonsimple_values_and_holders {
    void **values_and_holders;
    uint8_t *status;
};

just before struct instance { in detail/common.h, then using it in the union by changing the union from:

    union {
        void *simple_value_holder[1 + instance_simple_holder_in_ptrs()];
        struct { /* ... */ } nonsimple;
    };

to:

    union {
        void *simple_value_holder[1 + instance_simple_holder_in_ptrs()];
        nonsimple_values_and_holders nonsimple;
    };

(Assuming that's the only thing raising this warning, of course).

[Psirus]

Fine by me, it's just that most of the pybind code is way over my head and I didn't want to break anything.

[Psirus]

Solved in an improved way in #1248.