libcxx: Add missing implementation source files (#1089)

Added the implementation for `std::bad_any_cast` from upstream
`any.cpp`, and `std::bad_variant_access` from upstream `variant.cpp`.

This fixes missing `vtable` and `typeinfo` symbols when trying to link
code referencing these exception types.

third_party/libcxx/variant.cc

@@ -0,0 +1,17 @@
+//===------------------------ variant.cpp ---------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "third_party/libcxx/variant"
+
+namespace std {
+
+const char* bad_variant_access::what() const noexcept {
+  return "bad_variant_access";
+}
+
+}  // namespace std