Add Filesystem TS -- Complete

Add the completed std::experimental::filesystem implementation and tests.
The implementation supports C++11 or newer.

The TS is built as part of 'libc++experimental.a'. Users of the TS need to
manually link this library. Building and testing the TS can be disabled using
the CMake option '-DLIBCXX_ENABLE_FILESYSTEM=OFF'.

Currently 'libc++experimental.a' is not installed by default. To turn on the
installation of the library use '-DLIBCXX_INSTALL_EXPERIMENTAL_LIBRARY=ON'.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@273034 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/experimental/filesystem/class.path/synop.pass.cpp

@@ -0,0 +1,39 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+
+// <experimental/filesystem>
+
+// class path
+
+// typedef ... value_type;
+// typedef basic_string<value_type> string_type;
+// static constexpr value_type preferred_separator = ...;
+
+#include <experimental/filesystem>
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+
+namespace fs = std::experimental::filesystem;
+
+int main() {
+  using namespace fs;
+  ASSERT_SAME_TYPE(path::value_type, char);
+  ASSERT_SAME_TYPE(path::string_type, std::basic_string<path::value_type>);
+  {
+    ASSERT_SAME_TYPE(const path::value_type, decltype(path::preferred_separator));
+    static_assert(path::preferred_separator == '/', "");
+    // Make preferred_separator ODR used by taking it's address.
+    const char* dummy = &path::preferred_separator;
+    ((void)dummy);
+  }
+}