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/cpptools/cpptoolsplugin.h

@@ -133,6 +133,7 @@ private slots:
     void test_cppsourceprocessor_includes_cyclic();
     void test_cppsourceprocessor_includes_allDiagnostics();
     void test_cppsourceprocessor_macroUses();
+    void test_cppsourceprocessor_includeNext();
 
     void test_functionutils_virtualFunctions();
     void test_functionutils_virtualFunctions_data();