Export of internal Abseil changes

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

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

PiperOrigin-RevId: 306729491

--
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

--
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

--
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

absl/synchronization/mutex.h

@@ -331,17 +331,16 @@ class ABSL_LOCKABLE Mutex {
   // Mutex::AwaitWithTimeout()
   // Mutex::AwaitWithDeadline()
   //
-  // If `cond` is initially true, do nothing, or act as though `cond` is
-  // initially false.
-  //
-  // If `cond` is initially false, unlock this `Mutex` and block until
-  // simultaneously:
+  // Unlocks this `Mutex` and blocks until simultaneously:
   //   - either `cond` is true or the {timeout has expired, deadline has passed}
   //     and
   //   - this `Mutex` can be reacquired,
   // then reacquire this `Mutex` in the same mode in which it was previously
   // held, returning `true` iff `cond` is `true` on return.
   //
+  // If the condition is initially `true`, the implementation *may* skip the
+  // release/re-acquire step and return immediately.
+  //
   // Deadlines in the past are equivalent to an immediate deadline.
   // Negative timeouts are equivalent to a zero timeout.
   //