[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/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 997d213..77010b5 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -188,7 +188,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_barrier``                             ``201907L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_bind_front``                          *unimplemented*
+    ``__cpp_lib_bind_front``                          ``201907L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_bit_cast``                            *unimplemented*
     ------------------------------------------------- -----------------