Sink DwarfDebug::attachLowHighPC into DwarfCompileUnit

One of many things to sink down into DwarfCompileUnit to allow handling
of subprograms in both the skeleton and dwo CU under Fission.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219058 91177308-0d34-0410-b5e6-96231b3b80d8