[libc++] Implement `std::expected` P0323R12

Implement `std::expected` https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p0323r12.html

Added tests

Differential Revision: https://reviews.llvm.org/D124516

address comment

NOKEYCHECK=True
GitOrigin-RevId: e356f681f6c46ac35f933dc0cef3b25ceee8b210
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index a137126..c4ecbd6 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -318,6 +318,8 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_typeinfo``                  *unimplemented*
     ------------------------------------------------- -----------------
+    ``__cpp_lib_expected``                            ``202202L``
+    ------------------------------------------------- -----------------
     ``__cpp_lib_forward_like``                        ``202207L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_invoke_r``                            *unimplemented*