[libc++][AIX] AIX allows for changing permissions of symlinks

[ZarkoCA]

Summary

The test fails on AIX due to it expecting an error as on Linux. However, as on
other non-Linux systems symlinks permissions are supported so expect an empty
error code.

[daltenty]

LGTM, similarly to the other platforms indicated the bits can be set through the filesystem interface (though our chmod -h is a no-op)

[ZarkoCA]

Pinging this for the libc++ group

libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.permissions/permissions.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
 
-// XFAIL: LIBCXX-AIX-FIXME
-
 // <filesystem>
 
 // void permissions(const path& p, perms prms,
 //                  perm_options opts = perm_options::replace);
 // void permissions(const path& p, perms prms, std::error_code& ec) noexcept;
 // void permissions(const path& p, perms prms, perm_options opts, std::error_code);
 
 
struct TestCase {
                perm_options xopts = perm_options::replace)
           : set_perms(xperms), expected(xexpect), opts(xopts) {}
     } cases[] = {
         {perms::owner_all, perms::owner_all},
         {perms::group_all, perms::owner_all | perms::group_all, perm_options::add},
         {perms::owner_all, perms::group_all, perm_options::remove},
     };
     for (auto const& TC : cases) {
-#if defined(__APPLE__) || defined(__FreeBSD__) || defined(__NetBSD__)
+#if defined(__APPLE__) || defined(__FreeBSD__) || defined(__NetBSD__) || defined(_AIX)
         // On OS X symlink permissions are supported. We should get an empty
         // error code and the expected permissions.
         const auto expected_link_perms = TC.expected;
         std::error_code expected_ec;
 #else
         // On linux symlink permissions are not supported. The error code should
         // be 'operation_not_supported' and the symlink permissions should be
         // unchanged.