Cleanup DWARFCompileUnit and DWARFUnit in preparation for adding DWARFTypeUnit
Many things that were in DWARFCompileUnit actually need to be in DWARFUnit. This patch moves all DWARFUnit specific things over into DWARFUnit and fixes the layering. This is in preparation for adding DWARFTypeUnit for the .debug_types patch.
Differential Revision: https://reviews.llvm.org/D45170