C++: Finish gcc's include_next support

This implements the actual include_next logic and thus completes

    commit b934cc1
    C++: pass #include_next down to CppPreprocessor::tryIncludeFile

    commmit 140b502
    C++: Highlight argument to gcc's #include_next extension

Based on https://gcc.gnu.org/onlinedocs/cpp/Wrapper-Headers.html

Task-number: QTCREATORBUG-10225
Change-Id: I7eef7f5ea64a114f6d092304d32b72c55c2ce134
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>

src/plugins/coreplugin/testdatadir.h

@@ -52,8 +52,6 @@ class CORE_EXPORT TestDataDir
 public:
     TestDataDir(const QString &directory);
     QString file(const QString &fileName) const;
-
-protected:
     QString directory(const QString &subdir = QString(), bool clean = true) const;
 
 private: