[DWARF parser] Simplify and re-format a method

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207151 91177308-0d34-0410-b5e6-96231b3b80d8
[DWARF parser] DWARFUnit ctor doesn't need both parsed and raw .debug_abbrev section. Remove the former.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207153 91177308-0d34-0410-b5e6-96231b3b80d8