Running jextract on the following fails:

```
struct {
union { long sk; long :64; };
};
```

It prints `InvalidFieldName. segment: org.openjdk.jextract.clang.Type@bb5e20ff, fieldName: `
without any indication of the error location.

jextract is fine with the above example if we either add a property name for the union

```
struct {
union { long sk; long :64; } value;
};
```

or if we remove the width specification of the long:

```
struct {
union { long sk; long; };
};
```