[libc++] [P1065] Constexpr invoke, reference_wrapper, mem_fn, not_fn, default_searcher.

This completes the implementation of P1065 "constexpr INVOKE":
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1065r2.html

This doesn't yet complete the implementation of P1032 "Misc constexpr bits,"
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1032r1.html
but it does complete all of the <functional> bits, which means
that we can now set `__cpp_lib_constexpr_functional` for C++20.

This could use more constexpr tests for `std::reference_wrapper<T>`,
but the existing tests are extremely non-constexpr-friendly and
so I don't want to get into that rabbit-hole today.

Differential Revision: https://reviews.llvm.org/D93815

GitOrigin-RevId: 7b00e9fae3853d4693e608cc52f6d6da5059f5ff
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 091d4b7..5930cda 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -194,7 +194,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_dynamic_alloc``             ``201907L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_constexpr_misc``                      *unimplemented*
+    ``__cpp_lib_constexpr_functional``                ``201907L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_numeric``                   ``201911L``
     ------------------------------------------------- -----------------