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692d3df279e7592d01c1008cb85f2a010c3d17da by Abseil Team <absl-team@google.com>:

Use EXPECT_DEATH_IF_SUPPORTED instead of raw EXPECT_DEATH.

PiperOrigin-RevId: 307802196

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ebc40936b677b79cad9f87f944794c35946f9dbd by Gennadiy Rozental <rogeeff@google.com>:

Eliminate SetCallback from absl::Flag<T> public interface.

We also make SetCallback on FlagRegistrar to return rvalue, so that we can add more tail calls after it.

PiperOrigin-RevId: 307745935
GitOrigin-RevId: 15f69a9dae9c70c884ce85ca1a4bf359a2609db0
Change-Id: Ibec13463e44e4071c48fb12389f47e716cee7a9d