Export of internal Abseil changes.

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0c2e865b4611eda0b43e64c177ac34540f059be2 by Mark Barolak <mbar@google.com>:

Fix a sample code snippet that assumes `absl::string_view::const_iterator` is `const char*`.  This is generally true, however in C++17 builds, absl::string_view is an alias for std::string_view and on MSVC, the std::string_view::const_iterator is an object instead of just a pointer.

PiperOrigin-RevId: 233631168

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16f472f1b94dabf81632d6845574513546c8026e by Abseil Team <absl-team@google.com>:

Updating documentation for custom StrSplit delimiters to reflect
the fact that string_view does not have a (string_view, start, len)
constructor.

PiperOrigin-RevId: 233611745

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a3192dc5122a9a67e0e4d4d2421b0cdb10612c27 by Eric Fiselier <ericwf@google.com>:

Add missing <cassert> include.

PiperOrigin-RevId: 233523528
GitOrigin-RevId: 0c2e865b4611eda0b43e64c177ac34540f059be2
Change-Id: Ic12e547da3e9c4fe2e79b880f9426fa3cc0f4866