Export of internal Abseil changes.

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6fca451d74e509671f0996e15ea05008f73c9957 by Eric Fiselier <ericwf@google.com>:

Support vector<bool>::reference and ::const_reference
in absl::Substitute.

PiperOrigin-RevId: 248524270

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a4b298c74acb8ae0688ed681052593623d8021c7 by Abseil Team <absl-team@google.com>:

Clarify that a static `SpinLock` using the `LinkerInitialized` constructor is
initialized in non-cooperative mode.

PiperOrigin-RevId: 248386381
GitOrigin-RevId: 6fca451d74e509671f0996e15ea05008f73c9957
Change-Id: I13d54c2034695e7677170cdc7b86384b7d7d9cb5