[libc++] Update generate_feature_test_macro_components.py to match SD-6.

It's still a little confusing because in many cases C++17 and C++20
have different values, and libc++ implements the C++17 behavior but
not the C++20 behavior; 'unimplemented' can't represent that scenario.
Ultimately we probably ought to completely redesign the script to be
in terms of paper numbers, rather than language revisions, and make
it generate the CSV files like "Cxx2aStatusPaperStatus.csv" as well.

Most newly added macros are unimplemented. I've marked a few as implemented,
though, based on my reading of the code; for example I was pretty sure
`__cpp_lib_latch` is implemented since we have `<latch>`.

Differential Revision: https://reviews.llvm.org/D93830

Author: Arthur O'Dwyer

libcxx/docs/FeatureTestMacroTable.rst

@@ -170,6 +170,8 @@ Status
     -------------------------------------------------------------------
     ``__cpp_lib_array_constexpr``                     ``201811L``
     ------------------------------------------------- -----------------
+    ``__cpp_lib_assume_aligned``                      *unimplemented*
+    ------------------------------------------------- -----------------
     ``__cpp_lib_atomic_flag_test``                    ``201907L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_atomic_float``                        *unimplemented*
@@ -184,48 +186,108 @@ Status
     ------------------------------------------------- -----------------
     ``__cpp_lib_atomic_wait``                         ``201907L``
     ------------------------------------------------- -----------------
+    ``__cpp_lib_barrier``                             ``201907L``
+    ------------------------------------------------- -----------------
     ``__cpp_lib_bind_front``                          *unimplemented*
     ------------------------------------------------- -----------------
     ``__cpp_lib_bit_cast``                            *unimplemented*
     ------------------------------------------------- -----------------
+    ``__cpp_lib_bitops``                              *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_bounded_array_traits``                ``201902L``
+    ------------------------------------------------- -----------------
     ``__cpp_lib_char8_t``                             ``201811L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_concepts``                            *unimplemented*
     ------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_algorithms``                *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_complex``                   *unimplemented*
+    ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_dynamic_alloc``             ``201907L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_constexpr_functional``                ``201907L``
+    ``__cpp_lib_constexpr_functional``                *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_iterator``                  *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_memory``                    *unimplemented*
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_numeric``                   ``201911L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_constexpr_swap_algorithms``           *unimplemented*
+    ``__cpp_lib_constexpr_string``                    *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_string_view``               *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_tuple``                     *unimplemented*
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_utility``                   ``201811L``
     ------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_vector``                    *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_coroutine``                           *unimplemented*
+    ------------------------------------------------- -----------------
     ``__cpp_lib_destroying_delete``                   ``201806L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_endian``                              ``201907L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_erase_if``                            ``202002L``
     ------------------------------------------------- -----------------
+    ``__cpp_lib_execution``                           *unimplemented*
+    ------------------------------------------------- -----------------
     ``__cpp_lib_generic_unordered_lookup``            ``201811L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_int_pow2``                            ``202002L``
     ------------------------------------------------- -----------------
+    ``__cpp_lib_integer_comparison_functions``        *unimplemented*
+    ------------------------------------------------- -----------------
     ``__cpp_lib_interpolate``                         ``201902L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_is_constant_evaluated``               ``201811L``
     ------------------------------------------------- -----------------
+    ``__cpp_lib_is_layout_compatible``                *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_is_nothrow_convertible``              ``201806L``
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_is_pointer_interconvertible``         *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_jthread``                             *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_latch``                               ``201907L``
+    ------------------------------------------------- -----------------
     ``__cpp_lib_list_remove_return_type``             ``201806L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_math_constants``                      ``201907L``
     ------------------------------------------------- -----------------
+    ``__cpp_lib_polymorphic_allocator``               *unimplemented*
+    ------------------------------------------------- -----------------
     ``__cpp_lib_ranges``                              *unimplemented*
     ------------------------------------------------- -----------------
+    ``__cpp_lib_remove_cvref``                        ``201711L``
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_semaphore``                           ``201907L``
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_shift``                               *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_smart_ptr_for_overwrite``             *unimplemented*
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_source_location``                     *unimplemented*
+    ------------------------------------------------- -----------------
     ``__cpp_lib_span``                                ``202002L``
     ------------------------------------------------- -----------------
+    ``__cpp_lib_ssize``                               ``201902L``
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_starts_ends_with``                    ``201711L``
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_string_view``                         ``201803L``
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_syncbuf``                             *unimplemented*
+    ------------------------------------------------- -----------------
     ``__cpp_lib_three_way_comparison``                *unimplemented*
     ------------------------------------------------- -----------------
+    ``__cpp_lib_to_address``                          ``201711L``
+    ------------------------------------------------- -----------------
     ``__cpp_lib_to_array``                            ``201907L``
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_unwrap_ref``                          ``201811L``
     ================================================= =================