Export of internal Abseil changes.

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5da9755667df37e38ccaf6938c9f408e294110bb by Shaindel Schwartz <shaindel@google.com>:

Import of CCTZ from GitHub.

PiperOrigin-RevId: 232942734

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b6fb275769c66fdd2bd92b119198c59e9a7dd737 by Samuel Benzaquen <sbenza@google.com>:

Fix integral underflow when from-arg width is INT_MIN.

PiperOrigin-RevId: 232888037

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4135dbba4a26c4642277fc2a7e2a833d593daa1c by Abseil Team <absl-team@google.com>:

Add the insert_return_type alias to raw_hash_set.

PiperOrigin-RevId: 232683892

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0b120b7d3693800bbb886f6fc607ae54a9338cb1 by Abseil Team <absl-team@google.com>:

Macros to detect and disabled SafeStack
https://clang.llvm.org/docs/SafeStack.html

PiperOrigin-RevId: 232680114

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a77b3fb533a9e37966d1d6ef5ccd09c73fff2ca1 by Abseil Team <absl-team@google.com>:

Avoid potential red zone clobber

Pushing on the stack on x86-64 may clobber local variables held below
%rsp in the red zone. Avoid this by using lea on x86-64.

PiperOrigin-RevId: 232592478

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bf326a0eefa92f4e704287563df0c5a5b1873b6d by Eric Fiselier <ericwf@google.com>:

Add additional tests for AbslHashValue.

PiperOrigin-RevId: 232344325

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816e4f98fd7632c944c779db87b7dac4e138afcf by Eric Fiselier <ericwf@google.com>:

Avoid upcoming GCC 9.0 warnings about base class init.

Currently, in trunk, GCC has a new warning under -Wextra
that diagnoses when a derived class fails to explicitly
initialize the base class in a constructor initializer list.

This patch avoids this warning.

PiperOrigin-RevId: 232327626

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779c0f44b3c2b7a04d4bdf978641eb8180515bf6 by Eric Fiselier <ericwf@google.com>:

Guard against C++2a char8_t change.

PiperOrigin-RevId: 232326178

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41e5395b85bbbfb5bf418cc21b04ad4ccb15a284 by Eric Fiselier <ericwf@google.com>:

Avoid Clang Warning

PiperOrigin-RevId: 232138866
GitOrigin-RevId: 5da9755667df37e38ccaf6938c9f408e294110bb
Change-Id: I49ee4f58db177b81b039d7d949f671c97c5a7933