Revert "Cleanup DWARFCompileUnit and DWARFUnit in preparation for adding DWARFTypeUnit"

The reverted commit changed DWARFUnit from https://reviews.llvm.org/D40466 and
https://reviews.llvm.org/D42892 that was prepared for DWARFPartialUnit and
made from it a superclass for DWARFTypeUnit.  DWARFUnit's intention was:
	DWARFUnit->DWARFSomeNameUnit->DWARFCompileUnit
	DWARFUnit->DWARFSomeNameUnit->DWARFTypeUnit
	DWARFUnit->DWARFPartialUnit

Discussed at: https://reviews.llvm.org/D45170

This reverts commit r329305.

llvm-svn: 329423