The libDebugInfo DIE parsing doesn't store these relationships, we have to
recompute them. This commit introduces the CompileUnit bookkeeping class to
store this data. It will be expanded with more fields in the future.
No tests as this produces no visible output.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227382 91177308-0d34-0410-b5e6-96231b3b80d8