Add 'third_party/abseil_cpp/' from commit '768eb2ca28'

git-subtree-dir: third_party/abseil_cpp
git-subtree-mainline: ffb2ae54be
git-subtree-split: 768eb2ca28

third_party/abseil_cpp/absl/flags/declare.h

@@ -0,0 +1,66 @@
+//
+//  Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: declare.h
+// -----------------------------------------------------------------------------
+//
+// This file defines the ABSL_DECLARE_FLAG macro, allowing you to declare an
+// `absl::Flag` for use within a translation unit. You should place this
+// declaration within the header file associated with the .cc file that defines
+// and owns the `Flag`.
+
+#ifndef ABSL_FLAGS_DECLARE_H_
+#define ABSL_FLAGS_DECLARE_H_
+
+#include "absl/base/config.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace flags_internal {
+
+// absl::Flag<T> represents a flag of type 'T' created by ABSL_FLAG.
+template <typename T>
+class Flag;
+
+}  // namespace flags_internal
+
+// Flag
+//
+// Forward declaration of the `absl::Flag` type for use in defining the macro.
+#if defined(_MSC_VER) && !defined(__clang__)
+template <typename T>
+class Flag;
+#else
+template <typename T>
+using Flag = flags_internal::Flag<T>;
+#endif
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+// ABSL_DECLARE_FLAG()
+//
+// This macro is a convenience for declaring use of an `absl::Flag` within a
+// translation unit. This macro should be used within a header file to
+// declare usage of the flag within any .cc file including that header file.
+//
+// The ABSL_DECLARE_FLAG(type, name) macro expands to:
+//
+//   extern absl::Flag<type> FLAGS_name;
+#define ABSL_DECLARE_FLAG(type, name) extern ::absl::Flag<type> FLAGS_##name
+
+#endif  // ABSL_FLAGS_DECLARE_H_