Reintroduce accidentally deleted "protected" keyword in SymbolFileDWARF (#69990)

The "protected" was accidentally removed during refactoring of
SymbolFileDWARF. Reintroduce it and also make DWARFASTParser a friend
class since 040c4f4d98f3306e068521e3c218bdbc170f81f3 was merged and
won't build without it, as it dependeds on a method which was made
public by accident.