[libc++] [P1032] Misc constexpr bits in <iterator>, <string_view>, <tuple>, <utility>.

This completes the implementation of P1032's changes to <iterator>,
<string_view>, <tuple>, and <utility> in C++20.
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1032r1.html

Drive-by fix a couple of unintended rvalues in "*iterators*/*.fail.cpp".

Differential Revision: https://reviews.llvm.org/D96385

GitOrigin-RevId: 06e2b737aa0347b42e8bf37cb00a053eab0a9393
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 693a668..40a6bda 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -208,17 +208,17 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_functional``                ``201907L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_constexpr_iterator``                  *unimplemented*
+    ``__cpp_lib_constexpr_iterator``                  ``201811L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_memory``                    ``201811L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_numeric``                   ``201911L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_constexpr_string``                    *unimplemented*
+    ``__cpp_lib_constexpr_string``                    ``201811L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_constexpr_string_view``               *unimplemented*
+    ``__cpp_lib_constexpr_string_view``               ``201811L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_constexpr_tuple``                     *unimplemented*
+    ``__cpp_lib_constexpr_tuple``                     ``201811L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_utility``                   ``201811L``
     ------------------------------------------------- -----------------