merge(3p/absl): subtree merge of Abseil up to e19260f

... notably, this includes Abseil's own StatusOr type, which
conflicted with our implementation (that was taken from TensorFlow).

Change-Id: Ie7d6764b64055caaeb8dc7b6b9d066291e6b538f

third_party/abseil_cpp/absl/flags/commandlineflag.cc

@@ -30,9 +30,5 @@ bool CommandLineFlag::ParseFrom(absl::string_view value, std::string* error) {
                    flags_internal::kProgrammaticChange, *error);
 }
 
-namespace flags_internal {
-FlagStateInterface::~FlagStateInterface() {}
-}  // namespace flags_internal
 ABSL_NAMESPACE_END
 }  // namespace absl
-