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

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.

Differential Revision: https://reviews.llvm.org/D89678