Changes imported from Abseil "staging" branch:

- 46a4bd582619bfc4799c1a907316e914ed0c0105 Fixes to mutex_nonprod.  Apparently this has never built.... by Greg Falcon <gfalcon@google.com>
  - 4bcf6f013aeee53057527d70100de8535aec9b93 Add some comments to copts.bzl by Tom Manshreck <shreck@google.com>
  - 54cf717b1c5dfb814721f92a8eb9df5587f5f758 Internal change by Abseil Team <absl-team@google.com>
  - d1f1c7e6e61358300963b7bd50c203740a49297d Eliminate kFast{UInt,Int}{32,64}ToBufferSize constants by Jorg Brown <jorg@google.com>

GitOrigin-RevId: 46a4bd582619bfc4799c1a907316e914ed0c0105
Change-Id: I129b6b3d27d9ae2c6eb21d6e44a52d10399ef8aa

absl/synchronization/internal/mutex_nonprod.cc

@@ -231,6 +231,11 @@ bool Mutex::LockWhenWithTimeout(const Condition& cond, absl::Duration timeout) {
       cond, synchronization_internal::DeadlineFromTimeout(timeout));
 }
 
+void Mutex::ReaderLockWhen(const Condition& cond) {
+  ReaderLock();
+  Await(cond);
+}
+
 bool Mutex::ReaderLockWhenWithTimeout(const Condition& cond,
                                       absl::Duration timeout) {
   return LockWhenWithTimeout(cond, timeout);