Revert "[libc++] Build <filesystem> support as part of the dylib"

When I applied r356500 (https://reviews.llvm.org/D59152), I somehow
deleted all of filesystem's tests. I will revert r356500 and re-apply
it properly.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@356505 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/input.output/filesystems/fs.req.namespace/namespace.pass.cpp

@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// <filesystem>
+
+// namespace std::filesystem
+
+#include <filesystem>
+#include <type_traits>
+
+using namespace std::filesystem;
+
+int main(int, char**) {
+  static_assert(std::is_same<
+          path,
+          std::filesystem::path
+      >::value, "");
+
+  return 0;
+}