Export of internal Abseil changes

--
d447fdcb801036cf08197eece193a5a706661120 by Gennadiy Rozental <rogeeff@google.com>:

Eliminate the need for static function holding help message. This decreases the cost of ABSL_FLAG abstraction by 120 bytes under clang.

PiperOrigin-RevId: 281107806

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0aa6b91189f0e8b2381438c33465673a7ae02487 by Derek Mauro <dmauro@google.com>:

Disable the weak symbol CCTZ extension in the time test_util
on MinGW, which does not support it.

PiperOrigin-RevId: 280719769

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67322c41c3e776eb541de90fa4526bdb49422eb6 by Abseil Team <absl-team@google.com>:

Tune PeriodicSampler implementation (for internal-use only)

PiperOrigin-RevId: 280708943

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3a48c346340c7ed03816645cd327e1ff07729aa4 by Abseil Team <absl-team@google.com>:

Clean up public headers not to have warnings for "-Wcomma"

PiperOrigin-RevId: 280695373

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981acd1ef3b13a83a84f04f11c8931f4ed4451c9 by Matthew Brown <matthewbr@google.com>:

Release absl::int128.

PiperOrigin-RevId: 280690817

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d30fae9d2ec30b81322d2eb5afe7e13e45b4b422 by Derek Mauro <dmauro@google.com>:

Fix -Wundef warnings in random platform detection

PiperOrigin-RevId: 280669598
GitOrigin-RevId: d447fdcb801036cf08197eece193a5a706661120
Change-Id: Ie5e10e567c54b7de211833607689f233d4ddf734

absl/time/internal/test_util.cc

@@ -116,7 +116,10 @@ std::unique_ptr<cctz::ZoneInfoSource> TestFactory(
 
 }  // namespace
 
+#if !defined(__MINGW32__)
+// MinGW does not support the weak symbol extension mechanism.
 ZoneInfoSourceFactory zone_info_source_factory = TestFactory;
+#endif
 
 }  // namespace cctz_extension
 }  // namespace time_internal

absl/time/time.h

@@ -1412,8 +1412,7 @@ constexpr Duration FromInt64(int64_t v, std::ratio<3600>) {
 // IsValidRep64<T>(0) is true if the expression `int64_t{std::declval<T>()}` is
 // valid. That is, if a T can be assigned to an int64_t without narrowing.
 template <typename T>
-constexpr auto IsValidRep64(int)
-    -> decltype(int64_t{std::declval<T>()}, bool()) {
+constexpr auto IsValidRep64(int) -> decltype(int64_t{std::declval<T>()} == 0) {
   return true;
 }
 template <typename T>