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/clangtools/clangtoolruncontrol.cpp

@@ -194,6 +194,7 @@ static AnalyzeUnits toAnalyzeUnits(const FileInfos &fileInfos)
         CompilerOptionsBuilder optionsBuilder(*fileInfo.projectPart,
                                               CppTools::UseSystemHeader::No,
                                               CppTools::SkipBuiltIn::No,
+                                              CppTools::SkipLanguageDefines::Yes,
                                               QString(CLANG_VERSION),
                                               QString(CLANG_RESOURCE_DIR));
         QStringList arguments = extraClangToolsPrependOptions();