Export of internal Abseil changes

--
034c30a00c64d93b9fcbc9d99a0a33801544d741 by Gennadiy Rozental <rogeeff@google.com>:

Split private handle interfaces accessor into a separate target with private visibility.

PiperOrigin-RevId: 310391488

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6f6ca869309b17900b90849e08488ce7f7b0193a by Derek Mauro <dmauro@google.com>:

Remove __CLANG_SUPPORT_DYN_ANNOTATION__, which is a symbol defined by us
to be true in all builds

PiperOrigin-RevId: 310385325

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ed5c1880c86973c000e826a3006b38e53ab3ed52 by Samuel Benzaquen <sbenza@google.com>:

Add tests to exercise extreme width and precision, and fix the overflows from
it.

PiperOrigin-RevId: 310224957
GitOrigin-RevId: 034c30a00c64d93b9fcbc9d99a0a33801544d741
Change-Id: I6c89a3c89ae92fa617c696044148ce9a79bcdda8

absl/flags/internal/private_handle_accessor.h

@@ -0,0 +1,52 @@
+//
+// Copyright 2020 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.
+
+#ifndef ABSL_FLAGS_INTERNAL_PRIVATE_HANDLE_ACCESSOR_H_
+#define ABSL_FLAGS_INTERNAL_PRIVATE_HANDLE_ACCESSOR_H_
+
+#include "absl/flags/internal/commandlineflag.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace flags_internal {
+
+// This class serves as a trampoline to access private methods of
+// CommandLineFlag. This class is intended for use exclusively internally inside
+// of the Abseil Flags implementation.
+class PrivateHandleAccessor {
+ public:
+  // Access to CommandLineFlag::TypeId.
+  static FlagFastTypeId TypeId(const CommandLineFlag& flag);
+
+  // Access to CommandLineFlag::SaveState.
+  static std::unique_ptr<FlagStateInterface> SaveState(CommandLineFlag* flag);
+
+  // Access to CommandLineFlag::ValidateInputValue.
+  static bool ValidateInputValue(const CommandLineFlag& flag,
+                                 absl::string_view value);
+
+  // Access to CommandLineFlag::CheckDefaultValueParsingRoundtrip.
+  static void CheckDefaultValueParsingRoundtrip(const CommandLineFlag& flag);
+
+  static bool ParseFrom(CommandLineFlag* flag, absl::string_view value,
+                        flags_internal::FlagSettingMode set_mode,
+                        flags_internal::ValueSource source, std::string* error);
+};
+
+}  // namespace flags_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FLAGS_INTERNAL_PRIVATE_HANDLE_ACCESSOR_H_