Sink DwarfUnit::Skeleton down into DwarfCompileUnit

Type units no longer have skeletons and it's misleading to be able to
query for a type unit's skeleton (it might incorrectly lead one to
conclude that if a unit doesn't have a skeleton it's not in a .dwo
file... ).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221055 91177308-0d34-0410-b5e6-96231b3b80d8