[libc++] [P0887] Add newest feature-test macros; mark `type_identity` as implemented.

`__cpp_lib_type_identity` was implemented way back in cf49ccd0 (Clang 8),
probably before the feature-test macro had been settled on.

`__cpp_lib_string_resize_and_overwrite` will be added by D113013 so I didn't add it here.

Fixes #46605.

Differential Revision: https://reviews.llvm.org/D116433

NOKEYCHECK=True
GitOrigin-RevId: 855d7bedb71376fc5bdfc17644715b2fa3b10f46
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 0578ff8..714ed80 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -290,16 +290,36 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_to_array``                            ``201907L``
     ------------------------------------------------- -----------------
+    ``__cpp_lib_type_identity``                       ``201806L``
+    ------------------------------------------------- -----------------
     ``__cpp_lib_unwrap_ref``                          ``201811L``
     ------------------------------------------------- -----------------
     **C++ 2b**
     -------------------------------------------------------------------
+    ``__cpp_lib_allocate_at_least``                   *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_associative_heterogeneous_erasure``   *unimplemented*
+    ------------------------------------------------- -----------------
     ``__cpp_lib_byteswap``                            ``202110L``
     ------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_typeinfo``                  *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_invoke_r``                            *unimplemented*
+    ------------------------------------------------- -----------------
     ``__cpp_lib_is_scoped_enum``                      ``202011L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_monadic_optional``                    ``202110L``
     ------------------------------------------------- -----------------
+    ``__cpp_lib_move_only_function``                  *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_out_ptr``                             *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_ranges_starts_ends_with``             *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_ranges_zip``                          *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_spanstream``                          *unimplemented*
+    ------------------------------------------------- -----------------
     ``__cpp_lib_stacktrace``                          *unimplemented*
     ------------------------------------------------- -----------------
     ``__cpp_lib_stdatomic_h``                         *unimplemented*