[DebugInfo][NFCI] Add unittest for DWARFAbbreviationDeclarationSet

The classes relevant to DWARFDebugAbbrev do not have any unittests
verifying their behavior. Seeing as there is not much error handling
around these classes right now, I want to add some testing as I plan on
making changes to these classes in the near future.

Differential Revision: https://reviews.llvm.org/D151001