getClangVersion is a constant

Because getClangVersion never change and this avoid useless copies of strings

src/CppParser/CppParser.cpp

@@ -19,7 +19,7 @@ namespace CppSharp { namespace CppParser {
 
     CppParserOptions::~CppParserOptions() {}
 
-    std::string CppParserOptions::getClangVersion()
+    const std::string& CppParserOptions::getClangVersion()
     {
         return clangVersion;
     }
@@ -78,4 +78,4 @@ namespace CppSharp { namespace CppParser {
 
     ParserDiagnostic::~ParserDiagnostic() {}
 
-}} // namespace CppSharp::CppParser
+}} // namespace CppSharp::CppParser