Use absl::NoDestructor for global FlagRegistry.

PiperOrigin-RevId: 582861689
Change-Id: Iac257539ab282e9bfdc6d90e86c8da0c0d57b9c9

absl/flags/BUILD.bazel

@@ -190,6 +190,7 @@ cc_library(
         ":private_handle_accessor",
         "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:no_destructor",
         "//absl/container:flat_hash_map",
         "//absl/strings",
         "//absl/synchronization",

absl/flags/CMakeLists.txt

@@ -169,6 +169,7 @@ absl_cc_library(
     absl::strings
     absl::synchronization
     absl::flat_hash_map
+    absl::no_destructor
 )
 
 # Internal-only target, do not depend on directly.

absl/flags/reflection.cc

@@ -21,6 +21,7 @@
 #include <string>
 
 #include "absl/base/config.h"
+#include "absl/base/no_destructor.h"
 #include "absl/base/thread_annotations.h"
 #include "absl/container/flat_hash_map.h"
 #include "absl/flags/commandlineflag.h"
@@ -169,7 +170,7 @@ void FlagRegistry::RegisterFlag(CommandLineFlag& flag, const char* filename) {
 }
 
 FlagRegistry& FlagRegistry::GlobalRegistry() {
-  static FlagRegistry* global_registry = new FlagRegistry;
+  static absl::NoDestructor<FlagRegistry> global_registry;
   return *global_registry;
 }