[libcxx] [test] Allow fs::permissions(path, perms, perm_options, error_code) to be noexcept

[mstorsjo]

Summary

The standard doesn't declare this overload as noexcept, but doesn't either say that it strictly cannot be noexcept either. The function doesn't throw on errors that are signaled via error_code, but the standard says that it may throw a bad_alloc.

This fixes an error with libstdc++ on linux.

libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.permissions/permissions.pass.cpp

 {
     const path p; ((void)p);
     const perms pr{}; ((void)pr);
     const perm_options opts{}; ((void)opts);
     std::error_code ec; ((void)ec);
     ASSERT_NOT_NOEXCEPT(fs::permissions(p, pr));
     ASSERT_NOT_NOEXCEPT(fs::permissions(p, pr, opts));
     ASSERT_NOEXCEPT(fs::permissions(p, pr, ec));
-    ASSERT_NOT_NOEXCEPT(fs::permissions(p, pr, opts, ec));
+    LIBCPP_ONLY(ASSERT_NOT_NOEXCEPT(fs::permissions(p, pr, opts, ec)));
 }
 
 TEST_CASE(test_error_reporting)
 {
     auto checkThrow = [](path const& f, fs::perms opts,
                          const std::error_code& ec)
     {
 #ifndef TEST_HAS_NO_EXCEPTIONS