Implement LWG issue 2720. Replace perms::resolve_symlinks with perms::symlink_nofollow.

This changes how filesystem::permissions(p, perms) handles symlinks. Previously
symlinks were not resolved by default instead only getting resolved when
"perms::resolve_symlinks" was used. After this change symlinks are resolved
by default and perms::symlink_nofollow must be given to change this.

This issue has not yet been moved to Ready status, and I will revert if it
doesn't get moved at the current meeting. However I feel confident that it
will and it's nice to have implementations when moving issues.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@273328 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/experimental/filesystem/fs.enum/enum.perms.pass.cpp

@@ -63,6 +63,6 @@ int main() {
         E::unknown      == ME(0xFFFF) &&
         E::add_perms        == ME(0x10000) &&
         E::remove_perms     == ME(0x20000) &&
-        E::resolve_symlinks == ME(0x40000),
+        E::symlink_nofollow == ME(0x40000),
         "Expected enumeration values do not match");
 }