Replace std::atomic_flag with std::atomic<bool> to avoid the C++20

deprecation of ATOMIC_FLAG_INIT.

Another option would have been to use macros to only initialize
std::atomic_flag before C++20, but I decided to use one compilation
path instead.

The major difference between std::atomic_flag and std::atomic<bool> is
that the former is guaranteed to be lock-free, but we already assume
std::atomic<bool> is lock-free in many places.

https://en.cppreference.com/w/cpp/atomic/atomic_flag

PiperOrigin-RevId: 487397075
Change-Id: I3f1c539ec8b2ca58547282e69ed73e93243e8efe

absl/debugging/internal/stacktrace_x86-inl.inc

@@ -29,6 +29,7 @@
 #include <cstdint>
 #include <limits>
 
+#include "absl/base/attributes.h"
 #include "absl/base/macros.h"
 #include "absl/base/port.h"
 #include "absl/debugging/internal/address_is_readable.h"

absl/log/internal/log_message.cc

@@ -372,12 +372,16 @@ void LogMessage::Flush() {
   }
 
   // Have we already seen a fatal message?
-  ABSL_CONST_INIT static std::atomic_flag seen_fatal = ATOMIC_FLAG_INIT;
+  ABSL_CONST_INIT static std::atomic<bool> seen_fatal(false);
   if (data_->entry.log_severity() == absl::LogSeverity::kFatal &&
       absl::log_internal::ExitOnDFatal()) {
     // Exactly one LOG(FATAL) message is responsible for aborting the process,
     // even if multiple threads LOG(FATAL) concurrently.
-    data_->first_fatal = !seen_fatal.test_and_set(std::memory_order_relaxed);
+    bool expected_seen_fatal = false;
+    if (seen_fatal.compare_exchange_strong(expected_seen_fatal, true,
+                                           std::memory_order_relaxed)) {
+      data_->first_fatal = true;
+    }
   }
 
   data_->entry.text_message_with_prefix_and_newline_and_nul_ =