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.

llvm-svn: 273328
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 44e2ebadb2582936e4d6608e50ef234e22c89f87
diff --git a/include/experimental/filesystem b/include/experimental/filesystem
index be7d23f..7de93fd 100644
--- a/include/experimental/filesystem
+++ b/include/experimental/filesystem
@@ -292,7 +292,7 @@
 
     add_perms        = 0x10000,
     remove_perms     = 0x20000,
-    resolve_symlinks = 0x40000
+    symlink_nofollow = 0x40000
 };
 
 _LIBCPP_INLINE_VISIBILITY