[clang-tidy] make several member functions const

Found with readability-make-member-function-const

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Author: neheb

src/tag.cpp

@@ -29,7 +29,7 @@ Tag::Tag(const Token& token)
   }
 }
 
-std::string Tag::Translate(const Directives& directives) {
+std::string Tag::Translate(const Directives& directives) const {
   switch (type) {
     case VERBATIM:
       return value;

src/tag.h

@@ -23,7 +23,7 @@ struct Tag {
   };
 
   Tag(const Token& token);
-  std::string Translate(const Directives& directives);
+  std::string Translate(const Directives& directives) const;
 
   TYPE type;
   std::string handle, value;