[DWARFYAML][unittest] Refactor parseDWARFYAML().

In this change, `parseDWARFYAML()` is refactored to be able to parse
YAML decription into different data structures. We don't have to craft
the whole DWARF structure for a small test in the future.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D83220