[foreign] RFR 8217784: Ignore anonymous bitfields so they are handled as padding

[Jorn Vernee]

Hi,

 From the bug description:

Jextract fails on the following example:

```
struct Foo {
     unsigned int x: 1;
     int :7;
     unsigned int y: 8;
     int :16;
     int z;
};
```

The current code tries to look up the second field, but because it has 
no name this throws an illegal field name error.

---

I've fixed this by ignoring anonymous bitfields, since the 
StructLayoutComputer inserts padding automatically before the next 
field.

Please review the following.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217784
Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/8217784/webrev.00/

Thanks,
Jorn