Add constexpr to GetValue on ConstString holder.

src/ConstString.hpp

@@ -25,12 +25,8 @@ namespace Arbutils {
                 delete this;
             }
         }
-        inline void AddReference(){
+        inline void AddReference() { _references++; }
-            _references++;
+        inline constexpr const char* GetValue() const { return _value; }
-        }
-        inline const char* GetValue(){
-            return _value;
-        }
     };
 }