Implement filesystem::perm_options specified in NB comments.

The NB comments for filesystem changed permissions and added
a new enum `perm_options` which control how the permissions
are applied.

This implements than NB resolution

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@328476 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed