Exract most of DWARFCompileUnit into a new DWARFUnit to prepare for the coming DWARFTypeUnit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191233 91177308-0d34-0410-b5e6-96231b3b80d8
llvm-dwarfdump/libDebugInfo support for type units

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191234 91177308-0d34-0410-b5e6-96231b3b80d8