Make DwarfCompileUnit::Skeleton more narrowly typed (DwarfCompileUnit* instead of DwarfUnit*) now that it's specific to DwarfCompileUnit anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221060 91177308-0d34-0410-b5e6-96231b3b80d8