Don't remove __cplusplus

For the indexing we need all tool chain macros. Originally it was a fix
because the C++ version of the project part and __cplusplus could be
different but now they should be the same. They will be now removed in the
compiler options builder.

Change-Id: I7ae8721a29632473e76ecedb411a6c9001e5e199
Task-number: QTCREATORBUG-21265
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/clangcodemodel/clangutils.cpp

@@ -70,6 +70,7 @@ public:
         : CompilerOptionsBuilder(projectPart,
                                  UseSystemHeader::No,
                                  CppTools::SkipBuiltIn::No,
+                                 CppTools::SkipLanguageDefines::Yes,
                                  QString(CLANG_VERSION),
                                  QString(CLANG_RESOURCE_DIR))
     {