Implement P0430R2 - File system library on non-POSIX systems.

This patch implements P0430R2, who's largest change is adding the path::format
enumeration for supporting path format conversions in path constructors.

However, since libc++'s filesystem only really supports POSIX like systems,
there are no real changes needed. This patch simply adds the format enum
and then ignores it when it's passed to constructors.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@329031 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/experimental/filesystem/fs.enum/enum.path.format.pass.cpp

@@ -0,0 +1,38 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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;
+// enum class format;
+
+#include "filesystem_include.hpp"
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main() {
+  typedef fs::path::format E;
+  static_assert(std::is_enum<E>::value, "");
+
+  // Check that E is a scoped enum by checking for conversions.
+  typedef std::underlying_type<E>::type UT;
+  static_assert(!std::is_convertible<E, UT>::value, "");
+
+  LIBCPP_ONLY(static_assert(std::is_same<UT, unsigned char>::value, "")); // Implementation detail
+
+  static_assert(
+          E::auto_format   != E::native_format &&
+          E::auto_format   != E::generic_format &&
+          E::native_format != E::generic_format,
+        "Expected enumeration values are not unique");
+}