[libc++] Add bind_front function (P0356R5).

Implementes [[ http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0356r5.html | P0356R5 ]]. Adds `bind_front` to `functional`.

Reviewed By: ldionne, #libc, Quuxplusone

Differential Revision: https://reviews.llvm.org/D60368

GitOrigin-RevId: 84a50f5911bf8300cc4bea1f60673ede2145345b
diff --git a/include/version b/include/version
index e869723..49648a0 100644
--- a/include/version
+++ b/include/version
@@ -290,7 +290,7 @@
 # if !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_AVAILABILITY_DISABLE_FTM___cpp_lib_barrier)
 #   define __cpp_lib_barrier                            201907L
 # endif
-// # define __cpp_lib_bind_front                           201907L
+# define __cpp_lib_bind_front                           201907L
 // # define __cpp_lib_bit_cast                             201806L
 // # define __cpp_lib_bitops                               201907L
 # define __cpp_lib_bounded_array_traits                 201902L