DwarfDebug: Move ownership of CompileUnits into DwarfUnits

This avoids the need for an extra list of SkeletonCUs and associated
cleanup while staging things to be cleaner for further type unit
improvements.

Also hopefully fixes a memory leak introduced in r195166.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195536 91177308-0d34-0410-b5e6-96231b3b80d8