[DWARFYAML][debug_info] Make the 'Values' field optional.

This patch makes the 'Values' field optional. This is useful when we
handcraft the terminating entry of DIEs.

```
debug_info:
  - Version:  4
    ...
    Entries:
      - AbbrCode: 1
        Values:
          - Value: 0x1234
      - AbbrCode: 0 ## Termination
```

Reviewed By: jhenderson, grimar

Differential Revision: https://reviews.llvm.org/D85397