Export of internal Abseil changes

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35f516d528f4b53694ebe1f7debc023f1383cf4a by Shaindel Schwartz <shaindel@google.com>:

Internal change

PiperOrigin-RevId: 266967228

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40a0b91769133c48e3799a99f4dd2a7ce58bac91 by Derek Mauro <dmauro@google.com>:

Prevent absl::StrCat() and absl::StrAppend() from dereferencing std::string::end()

Fixes #374

PiperOrigin-RevId: 266447391
GitOrigin-RevId: 35f516d528f4b53694ebe1f7debc023f1383cf4a
Change-Id: I82e3a1bec5fa528db90a2f67dd3bc000e8dca8ab