Revert "DwarfUnit: Move the DICompileUnit Node to the DwarfCompileUnit only"

This reverts commit r198830.

Decided to go a different way with this...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198841 91177308-0d34-0410-b5e6-96231b3b80d8