Fix some occurrences of missing override keywords

Fix issues diagnosed by clang-tidy [modernize-use-override].

These occurrences are only showing up on OSX.
Signed-off-by: Matthias Maennich <matthias@maennich.net>

Source/cmGlobalXCodeGenerator.cxx

@@ -47,8 +47,11 @@ public:
     : Version("1.5")
   {
   }
-  void StartElement(const std::string&, const char**) { this->Data = ""; }
-  void EndElement(const std::string& name)
+  void StartElement(const std::string&, const char**) override
+  {
+    this->Data = "";
+  }
+  void EndElement(const std::string& name) override
   {
     if (name == "key") {
       this->Key = this->Data;
@@ -58,7 +61,7 @@ public:
       }
     }
   }
-  void CharacterDataHandler(const char* data, int length)
+  void CharacterDataHandler(const char* data, int length) override
   {
     this->Data.append(data, length);
   }

Source/cmLocalXCodeGenerator.h

@@ -27,16 +27,16 @@ public:
   ///! Set cache only and recurse to false by default.
   cmLocalXCodeGenerator(cmGlobalGenerator* gg, cmMakefile* mf);
 
-  virtual ~cmLocalXCodeGenerator();
-  virtual std::string GetTargetDirectory(
-    cmGeneratorTarget const* target) const;
-  virtual void AppendFlagEscape(std::string& flags,
-                                const std::string& rawFlag);
-  virtual void Generate();
+  ~cmLocalXCodeGenerator() override;
+  std::string GetTargetDirectory(
+    cmGeneratorTarget const* target) const override;
+  void AppendFlagEscape(std::string& flags,
+                        const std::string& rawFlag) override;
+  void Generate() override;
   virtual void GenerateInstallRules();
-  virtual void ComputeObjectFilenames(
+  void ComputeObjectFilenames(
     std::map<cmSourceFile const*, std::string>& mapping,
-    cmGeneratorTarget const* gt = nullptr);
+    cmGeneratorTarget const* gt = nullptr) override;
 
 private:
 };

Source/cmMachO.cxx

@@ -134,7 +134,7 @@ public:
     : cmMachOHeaderAndLoadCommands(_swap)
   {
   }
-  bool read_mach_o(cmsys::ifstream& fin)
+  bool read_mach_o(cmsys::ifstream& fin) override
   {
     if (!read(fin, this->Header)) {
       return false;

Source/cmXCode21Object.h

@@ -14,7 +14,7 @@ class cmXCode21Object : public cmXCodeObject
 {
 public:
   cmXCode21Object(PBXType ptype, Type type);
-  virtual void PrintComment(std::ostream&);
+  void PrintComment(std::ostream&) override;
   static void PrintList(std::vector<cmXCodeObject*> const&, std::ostream& out,
                         PBXType t);
   static void PrintList(std::vector<cmXCodeObject*> const&, std::ostream& out);