Roll libc++ from 26cbb86c20fe to 823696f806f8 (8 revisions)

https://chromium.googlesource.com/external/github.com/llvm/llvm-project/libcxx.git/+log/26cbb86c20fe..823696f806f8

2023-09-26 alexrichardson@google.com [libc++][lit] Allow overriding the executor for tests (#66545)
2023-09-26 ldionne.2@gmail.com [libc++] Use _Lazy in tuple constructors
2023-09-25 ldionne.2@gmail.com [libc++] Remove the CI job testing Clang 15 (#66406)
2023-09-25 ldionne.2@gmail.com [libc++] Remove the libc++ specific clang-format job (#67344)
2023-09-25 ldionne.2@gmail.com [libc++][NFC] Qualify mention of va_list with std:: in documentation
2023-09-25 ldionne.2@gmail.com [libc++] Add missing files to ignore_format.txt
2023-09-25 ldionne.2@gmail.com [libc++] Refactor tests for std::pointer_traits (#66645)
2023-09-25 aaron@aaronballman.com Revert "[libc++][ranges] Add benchmarks for the `from_range` constructors of `vector` and `deque`."

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Tbr: hans@chromium.org,thakis@chromium.org
Change-Id: Iac884519bad091048b5eb422daaf2315fe7ff8b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4893304
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Cr-Commit-Position: refs/heads/main@{#1201738}

• DEPS[diff]
• buildtools/deps_revisions.gni[diff]
• third_party/libc++/src[diff]