Export of internal Abseil changes

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0e867881e4b9f388a13d6fa8ed715192460130ab by Abseil Team <absl-team@google.com>:

Minor wording change to header comment for Mutex::AwaitWithDeadline(). No functional changes.

PiperOrigin-RevId: 306729491

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fc64361fb831003fa5e6fbb84a9a89338fd2838c by Derek Mauro <dmauro@google.com>:

Uses C++20 compatible allocator traits in Abseil types

This merges both instances of CountingAllocator in the Abseil codebase.
Makes the presubmits test C++20 mode.

Fixes #651

PiperOrigin-RevId: 306728102

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d759e5681b9dd6b7339fc019ed58fb5fdececdc3 by Derek Mauro <dmauro@google.com>:

Makes btree's iterator comparisons C++20 compatible

See https://stackoverflow.com/questions/60386792/c20-comparison-warning-about-ambiguous-reversed-operator

PiperOrigin-RevId: 306702048

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e9da5f409bc5ddb1bad308f9d8c41213c67a1d1e by Derek Mauro <dmauro@google.com>:

Switch a few uses of at() that should have been data() in the implementation of InlinedVector.
Use ABSL_HARDENING_ASSERT in resize().

PiperOrigin-RevId: 306670992
GitOrigin-RevId: 0e867881e4b9f388a13d6fa8ed715192460130ab
Change-Id: If431f3e5d77097e9901654773552dcc01dface87

ci/linux_clang-latest_libstdcxx_bazel.sh

@@ -25,7 +25,7 @@ if [[ -z ${ABSEIL_ROOT:-} ]]; then
 fi
 
 if [[ -z ${STD:-} ]]; then
-  STD="c++11 c++14 c++17"
+  STD="c++11 c++14 c++17 c++20"
 fi
 
 if [[ -z ${COMPILATION_MODE:-} ]]; then