[libcxx] [test] Mark filesystem braced_init test as requiring libc++

[mstorsjo]

Summary

This test fails on both MS STL and libstdc++, with errors like these:

error: use of overloaded operator '=' is ambiguous (with operand types 'std::filesystem::path' and 'void')

p = {};
~ ^ ~~

note: candidate function

path& operator=(path&&) noexcept = default;

note: candidate function

path& operator=(const path&) = default;

note: candidate function

path& operator=(string_type&& _Source) noexcept /* strengthened */ {

I'm not really familiar with these bits of modern C++ though to see what differs to libc++'s header, where this test succeeds, and whether that aspect is mandated by the standard though.

[mstorsjo]

Superseded by D95975.

libcxx/test/std/input.output/filesystems/class.path/path.member/path.assign/braced_init.pass.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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// REQUIRES: libc++
+
 // <filesystem>
 
 // class path
 
 // path& operator=(path const&);
 
 #include "filesystem_include.h"
 #include <type_traits>