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
diff --git a/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARFDebugMap.h b/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARFDebugMap.h
index 04599eb..dcca426 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARFDebugMap.h
+++ b/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARFDebugMap.h
@@ -136,7 +136,7 @@
   friend class DebugMapModule;
   friend struct DIERef;
   friend class DWARFASTParserClang;
-  friend class DWARFUnit;
+  friend class DWARFCompileUnit;
   friend class SymbolFileDWARF;
   struct OSOInfo {
     lldb::ModuleSP module_sp;