[ObjectYAML] NFC. Refactor DWARFYAML CompileUnit dump code

Summary: This patch refactors the DWARFYAML code for dumping compile units to use a visitor pattern. Using this design will, in the future, enable the DWARF YAML code to perform analysis and mutations of the DWARF DIEs. An example of such mutations would be calculating the length of a compile unit and updating the CU's Length field before writing the DIE. This support will make it easier to craft or modify DWARF tests by hand.

Reviewers: lhames

Subscribers: mgorny, fhahn, jgosnell, aprantl, llvm-commits

Differential Revision: https://reviews.llvm.org/D30357

llvm-svn: 297067