[DWARF] [ObjectYAML] Adding APIs for unittesting

Summary: This patch adds some new APIs to enable using the YAML DWARF representation in unit tests. The most basic new API is DWARFYAML::EmitDebugSections which converts a YAML string into a series of owned MemoryBuffer objects stored in a StringMap. The string map can then be used to construct a DWARFContext for parsing in place of an ObjectFile.

Reviewers: dblaikie, clayborg

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

Differential Revision: https://reviews.llvm.org/D28828

llvm-svn: 292634