Now the codebase can use the DWARFUnit superclass. It will make it later seamlessly work also with DWARFPartialUnit for DWZ.
This patch is only a search-and-replace easily undone, nothing interesting in it.
All DWZ patches are also applied in: git clone -b dwz git://git.jankratochvil.net/lldb
DWARFCompileUnit inherits from this class. This isn't needed right?