[libc++] Implement missing feature-test macro __cpp_lib_shared_ptr_arrays

This was forgotten when we implemented support for arrays in std::shared_ptr
in https://reviews.llvm.org/D62259.

GitOrigin-RevId: a65dc08d105e147c98ef17a47b9504210f6058c5
diff --git a/include/version b/include/version
index 2f1fd92..e32f47a 100644
--- a/include/version
+++ b/include/version
@@ -210,7 +210,7 @@
 # if !defined(_LIBCPP_HAS_NO_THREADS)
 #   define __cpp_lib_shared_mutex                       201505L
 # endif
-// # define __cpp_lib_shared_ptr_arrays                    201611L
+# define __cpp_lib_shared_ptr_arrays                    201611L
 # define __cpp_lib_shared_ptr_weak_type                 201606L
 # define __cpp_lib_string_view                          201606L
 // # define __cpp_lib_to_chars                             201611L