merge(3p/absl): subtree merge of Abseil up to e19260f

... notably, this includes Abseil's own StatusOr type, which
conflicted with our implementation (that was taken from TensorFlow).

Change-Id: Ie7d6764b64055caaeb8dc7b6b9d066291e6b538f

third_party/abseil_cpp/.github/ISSUE_TEMPLATE/00-bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: 'bug'
+assignees: ''
+---
+
+**Describe the bug**
+
+Include a clear and concise description of what the problem is, including what
+you expected to happen, and what actually happened.
+
+**Steps to reproduce the bug**
+
+It's important that we are able to reproduce the problem that you are
+experiencing. Please provide all code and relevant steps to reproduce the
+problem, including your `BUILD`/`CMakeLists.txt` file and build commands. Links
+to a GitHub branch or [godbolt.org](https://godbolt.org/) that demonstrate the
+problem are also helpful.
+
+**What version of Abseil are you using?**
+
+**What operating system and version are you using**
+
+If you are using a Linux distribution please include the name and version of the
+distribution as well.
+
+**What compiler and version are you using?**
+
+Please include the output of `gcc -v` or `clang -v`, or the equivalent for your
+compiler.
+
+**What build system are you using?**
+
+Please include the output of `bazel --version` or `cmake --version`, or the
+equivalent for your build system.
+
+**Additional context**
+
+Add any other context about the problem here.

third_party/abseil_cpp/.github/ISSUE_TEMPLATE/90-question.md

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+---
+name: Question
+about: Have a question? Ask us anything! :-)
+title: ''
+labels: 'question'
+assignees: ''
+---

third_party/abseil_cpp/.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enables: true