[libc++] [P0646] Add feature-test macro for __cpp_lib_list_remove_return_type.

Summary: The return type modification has already been implemented in rL364840 and rL365290.

Reviewers: ldionne, mclow.lists, EricWF, #libc!

Reviewed By: ldionne

Subscribers: christof, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D70275

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 05343588e3bf126319e166e55519531d29d08ba6
diff --git a/include/version b/include/version
index 2d9a2b3..ab62a85 100644
--- a/include/version
+++ b/include/version
@@ -232,7 +232,7 @@
 # if !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
 #   define __cpp_lib_is_constant_evaluated              201811L
 # endif
-// # define __cpp_lib_list_remove_return_type              201806L
+# define __cpp_lib_list_remove_return_type              201806L
 // # define __cpp_lib_ranges                               201811L
 # define __cpp_lib_to_array                                201907L
 // # define __cpp_lib_three_way_comparison                 201711L