Add libc++ and libc++abi to third-party.

third_party/libc++abi/src/cxa_aux_runtime.cpp

+//===------------------------ cxa_aux_runtime.cpp -------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//
+// This file implements the "Auxiliary Runtime APIs"
+// http://www.codesourcery.com/public/cxx-abi/abi-eh.html#cxx-aux
+//===----------------------------------------------------------------------===//
+
+#include "cxxabi.h"
+#include <typeinfo>
+
+namespace __cxxabiv1
+{
+
+extern "C"
+{
+
+LIBCXXABI_NORETURN
+void __cxa_bad_cast (void) {
+    throw std::bad_cast();
+}
+
+LIBCXXABI_NORETURN
+void __cxa_bad_typeid(void) {
+    throw std::bad_typeid();
+}
+
+}  // extern "C"
+
+}  // abi