Remove #include that the cp-to-llvm script added.

This is a short-term fix to make the buildbots happy. We should also fix
the script.

Author: zygoloid

llvm/include/llvm/Demangle/ItaniumDemangle.h

@@ -19,7 +19,6 @@
 #include "DemangleConfig.h"
 #include "StringViewExtras.h"
 #include "Utility.h"
-#include <__cxxabi_config.h>
 #include <algorithm>
 #include <cassert>
 #include <cctype>