Undo deprecate and inline of absl::optional and absl::nullopt

PiperOrigin-RevId: 916628855
Change-Id: Iadbb9574d526aa1b6d65992135de21529600fc01

absl/types/optional.h

diff --git a/absl/types/optional.h b/absl/types/optional.h
index eb225bf..c70efb1 100644
--- a/absl/types/optional.h
+++ b/absl/types/optional.h
@@ -56,16 +56,12 @@
   return std::make_optional<T>(il, std::forward<Args>(args)...);
 }
 
-inline constexpr const std::nullopt_t& nullopt ABSL_REFACTOR_INLINE
-    = std::nullopt;
+using std::nullopt;
 
 using nullopt_t ABSL_REFACTOR_INLINE
     = std::nullopt_t;
 
-template <typename T>
-using optional ABSL_REFACTOR_INLINE
-    = std::optional<T>;
-
+using std::optional;
 ABSL_NAMESPACE_END
 }  // namespace absl