We should detect and skip over anonymous bitfields

Given

```c
struct __attribute__((packed)) foo {
  int64_t x1 : 50;
  int64_t : 0;
  int64_t x2 : 50;
};
```

we currently try to generate a field for that anonymous bitfield (which is sometimes used to influence how the compiler packs the fields). With `--enable-record-dot` that then results in a name mangler failure (since it cannot mangle an empty name).