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/synchronization/CMakeLists.txt

@@ -52,7 +52,7 @@ absl_cc_library(
     "barrier.h"
     "blocking_counter.h"
     "internal/create_thread_identity.h"
-    "internal/mutex_nonprod.inc"
+    "internal/futex.h"
     "internal/per_thread_sem.h"
     "internal/waiter.h"
     "mutex.h"
@@ -149,6 +149,7 @@ absl_cc_test(
     absl::synchronization
     absl::thread_pool
     absl::base
+    absl::config
     absl::core_headers
     absl::memory
     absl::raw_logging_internal
@@ -179,6 +180,7 @@ absl_cc_library(
   DEPS
     absl::synchronization
     absl::base
+    absl::config
     absl::strings
     absl::time
     gmock