Reintroduce accidentally deleted "protected" keyword in SymbolFileDWARF

The "protected" was accidentally removed during refactoring of
SymbolFileDWARF. Reintroduce it and also make DWARFASTParser a friend
class since 040c4f4 was merged and
won't build without it, as it dependeds on a method which was made
public by accident.

(cherry picked from commit db9023e)

Author: augusto2112

lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.h

@@ -80,6 +80,7 @@ class SymbolFileDWARF : public lldb_private::SymbolFileCommon {
   friend class DebugMapModule;
   friend class DWARFCompileUnit;
   friend class DWARFDIE;
+  friend class DWARFASTParser;
   friend class DWARFASTParserClang;
   friend class DWARFASTParserSwift;