[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

Author: higuoxing

llvm/lib/ObjectYAML/DWARFEmitter.cpp

@@ -440,36 +440,36 @@ class DIEFixupVisitor : public DWARFYAML::Visitor {
 public:
   DIEFixupVisitor(DWARFYAML::Data &DI) : DWARFYAML::Visitor(DI){};
 
-private:
-  virtual void onStartCompileUnit(DWARFYAML::Unit &CU) {
+protected:
+  void onStartCompileUnit(DWARFYAML::Unit &CU) override {
     // Size of the unit header, excluding the length field itself.
     Length = CU.Version >= 5 ? 8 : 7;
   }
 
-  virtual void onEndCompileUnit(DWARFYAML::Unit &CU) { CU.Length = Length; }
+  void onEndCompileUnit(DWARFYAML::Unit &CU) override { CU.Length = Length; }
 
-  virtual void onStartDIE(DWARFYAML::Unit &CU, DWARFYAML::Entry &DIE) {
+  void onStartDIE(DWARFYAML::Unit &CU, DWARFYAML::Entry &DIE) override {
     Length += getULEB128Size(DIE.AbbrCode);
   }
 
-  virtual void onValue(const uint8_t U) { Length += 1; }
-  virtual void onValue(const uint16_t U) { Length += 2; }
-  virtual void onValue(const uint32_t U) { Length += 4; }
-  virtual void onValue(const uint64_t U, const bool LEB = false) {
+  void onValue(const uint8_t U) override { Length += 1; }
+  void onValue(const uint16_t U) override { Length += 2; }
+  void onValue(const uint32_t U) override { Length += 4; }
+  void onValue(const uint64_t U, const bool LEB = false) override {
     if (LEB)
       Length += getULEB128Size(U);
     else
       Length += 8;
   }
-  virtual void onValue(const int64_t S, const bool LEB = false) {
+  void onValue(const int64_t S, const bool LEB = false) override {
     if (LEB)
       Length += getSLEB128Size(S);
     else
       Length += 8;
   }
-  virtual void onValue(const StringRef String) { Length += String.size() + 1; }
+  void onValue(const StringRef String) override { Length += String.size() + 1; }
 
-  virtual void onValue(const MemoryBufferRef MBR) {
+  void onValue(const MemoryBufferRef MBR) override {
     Length += MBR.getBufferSize();
   }
 };