Export of internal Abseil changes.

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1edfe05ddddca43e7650b2d790df7c8498c0e588 by Abseil Team <absl-team@google.com>:

Adding an assert to catch various misuses of std::optional.

PiperOrigin-RevId: 249427865

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45463bbb7e59dfbc584b2f024368a63db98bd7a8 by CJ Johnson <johnsoncj@google.com>:

Migrates internal member function GetAllocator() to GetAllocPtr() and changes the return type to pointer instead of reference to avoid unnecessary copy in DestroyElements(...)

PiperOrigin-RevId: 249319571

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507835be22af85676143ee0c43a80a52bc32094c by Abseil Team <absl-team@google.com>:

Fix -Wstring-conversion in GetEnvVar (Windows implementation).

PiperOrigin-RevId: 249201897
GitOrigin-RevId: 1edfe05ddddca43e7650b2d790df7c8498c0e588
Change-Id: I9300131887ee507cf80d399c724cf87341e4f11a