[DWARFYAML] Add support for emitting custom range list content.

This patch adds support for emitting custom range list content.

We are able to handcraft a custom range list via the following syntax.

```
debug_rnglists:
  - Lists:
      - Entries:
          - Operator: DW_RLE_startx_endx
            Values:   [ 0x1234, 0x1234 ]
      - Content: '1234567890abcdef'
      - Content: 'abcdef1234567890'
```

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D84618