Deprecate and inline absl::optional and absl::nullopt

PiperOrigin-RevId: 916205787
Change-Id: I3b9e732184f45fc33f7c9fc3976c27b45bce81f9

absl/types/optional.h

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