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

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

+//===------------------------- abort_message.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 <stdlib.h>
+#include <stdio.h>
+#include <stdarg.h>
+#include "abort_message.h"
+
+#pragma GCC visibility push(hidden)
+
+#if __APPLE__ 
+#   if defined(__has_include) && __has_include(<CrashReporterClient.h>)
+#       define HAVE_CRASHREPORTERCLIENT_H 1
+#       include <CrashReporterClient.h>
+#   endif
+#endif
+
+__attribute__((visibility("hidden"), noreturn))
+void abort_message(const char* format, ...)
+{
+    // write message to stderr
+#if __APPLE__
+    fprintf(stderr, "libc++abi.dylib: ");
+#endif
+    va_list list;
+    va_start(list, format);
+    vfprintf(stderr, format, list);
+    va_end(list);
+    fprintf(stderr, "\n");
+    
+#if __APPLE__ && HAVE_CRASHREPORTERCLIENT_H
+    // record message in crash report
+    char* buffer;
+    va_list list2;
+    va_start(list2, format);
+    vasprintf(&buffer, format, list2);
+    va_end(list2);
+    CRSetCrashLogMessage(buffer);
+#endif
+
+    abort();
+}
+
+#pragma GCC visibility pop