Implement LWG2664 and update its status

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@284310 91177308-0d34-0410-b5e6-96231b3b80d8

include/experimental/filesystem

diff --git a/include/experimental/filesystem b/include/experimental/filesystem
index 10590a8..45de0d1 100644
--- a/include/experimental/filesystem
+++ b/include/experimental/filesystem
@@ -759,6 +759,8 @@
 public:
     // appends
     path& operator/=(const path& __p) {
+        _LIBCPP_ASSERT(!__p.has_root_name(),
+                      "cannot append to a path with a root name");
         __append_sep_if_needed(__p.empty() ? char{} : __p.__pn_[0]);
         __pn_ += __p.native();
         return *this;