Create a stub for DWARF parser unittests

Moves one DWARF-specific header to include/llvm/DebugInfo from lib/.
Add a short unittest for r179095.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179678 91177308-0d34-0410-b5e6-96231b3b80d8