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cbd86b318ce206d1fe9eb001979b8d2efe57e5fa by Derek Mauro <dmauro@google.com>:

Stop inlining `CurrentThreadIdentityIfPresent()` on Apple platforms

In some build configurations the Apple linker rejects thread_local
variables exposed in headers

Fixes #954
Fixes #965

PiperOrigin-RevId: 375930997
GitOrigin-RevId: e34600e09f748b686d20a4f729ae914ebfd9e7bf
Change-Id: I9fcd2f02bd6abdea3ed412217e1be18f2c7bf762