extend `-Wnonnull` diagnostics for `returns_nonnull` function attribute

[firewave]

static char* cb()
{
    return nullptr;
}

[[gnu::returns_nonnull]] static char* f()
{
    return cb();
}

int main()
{
    f();
}

https://godbolt.org/z/d9dz8dxE9

[firewave]

Not sure if this would still be in the scope of the diagnostics:

char* cb();

[[gnu::returns_nonnull]] static char* f()
{
    return cb();
}

int main()
{
    f();
}

I think it should at least try to leverage the lack of function attributes.