Implement LWG 2711. Constrain path members.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@279945 91177308-0d34-0410-b5e6-96231b3b80d8

include/experimental/filesystem

diff --git a/include/experimental/filesystem b/include/experimental/filesystem
index 7211c6b..fa2face 100644
--- a/include/experimental/filesystem
+++ b/include/experimental/filesystem
@@ -453,6 +453,9 @@
 template <class _Tp> struct __can_convert_char {
   static const bool value = false;
 };
+template <class _Tp> struct __can_convert_char<const _Tp>
+    : public __can_convert_char<_Tp> {
+};
 template <> struct __can_convert_char<char> {
     static const bool value = true;
     using __char_type = char;