Implement C++17 <variant>. Patch from Michael Park!

This patch was reviewed as https://reviews.llvm.org/D23263.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@288547 91177308-0d34-0410-b5e6-96231b3b80d8

src/variant.cpp

@@ -0,0 +1,18 @@
+//===------------------------ variant.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 "variant"
+
+namespace std {
+
+const char* bad_variant_access::what() const noexcept {
+  return "bad_variant_access";
+}
+
+}  // namespace std