Update clangVersion to const reference

Change clangVersion to a constant reference:
const std::string& clangVersion const;

src/CppParser/CppParser.h

@@ -51,7 +51,7 @@ namespace CppSharp { namespace CppParser {
         bool skipFunctionBodies = true;
 
     private:
-        std::string clangVersion;
+        const std::string& clangVersion const;
     };
 
     struct CS_API CppLinkerOptions
@@ -132,4 +132,4 @@ namespace CppSharp { namespace CppParser {
                                   bool Last);
     };
 
-}} // namespace CppSharp::CppParser
+}} // namespace CppSharp::CppParser