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

third_party/libc++/src/optional.cpp

+//===------------------------ optional.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 "optional"
+
+namespace std  // purposefully not using versioning namespace
+{
+
+#ifdef _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS
+
+bad_optional_access::~bad_optional_access() _NOEXCEPT {}
+
+#else
+
+bad_optional_access::~bad_optional_access() _NOEXCEPT = default;
+
+#endif
+
+}  // std