[DWARFYAML] Use override instead of virtual for better safety.

Functions in DWARFYML::FixupVisitor are declared as
virtual functions in its base class DWARFYAML::Visitor.
We should use the mordern "override" keyword instead
of "virtual" for virtual functions in subclasses for
better safety.

Besides, the visibility is changed from private to
protected to make it consistent with
DWARFYAML::FixupVisitor class and DWARFYAML::Visitor
class.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D83452