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

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

+//===-------------------------- cxa_virtual.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.
+//
+//===----------------------------------------------------------------------===//
+
+#include "cxxabi.h"
+#include "abort_message.h"
+
+namespace __cxxabiv1
+{
+
+extern "C"
+{
+
+LIBCXXABI_NORETURN
+void __cxa_pure_virtual(void) {
+    abort_message("Pure virtual function called!");
+}
+
+LIBCXXABI_NORETURN
+void __cxa_deleted_virtual(void) {
+    abort_message("Deleted virtual function called!");
+}
+
+}  // extern "C"
+
+}  // abi