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

[Higuoxing]

Summary

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

[grimar]

LGTM. Though better wait for a second opinion.

llvm/test/tools/yaml2obj/ELF/DWARF/debug-info.yaml

846

the 'Values' entry -> the Values` key for an entry` I guess is more clear.
(At first I thought hat Values is a property of debug_info.)

[jhenderson]

LGTM too.

[Higuoxing]

Address review comments.

Thanks for reviewing!