Fixes the initialization of fixed_address_empty_string

This makes `fixed_address_empty_string` a function returning a static instance.
It avoids the static-initialization fiasco that can happen when using a global static variable.

Resolves: protocolbuffers#8129

src/google/protobuf/generated_message_util.cc

@@ -72,15 +72,15 @@ void DestroyString(const void* s) {
   static_cast<const std::string*>(s)->~basic_string();
 }
 
-PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT
-    PROTOBUF_ATTRIBUTE_INIT_PRIORITY EmptyString
-        fixed_address_empty_string;  // NOLINT
-
+PROTOBUF_CONSTINIT const EmptyString& fixed_address_empty_string() {
+  PROTOBUF_CONSTINIT static const EmptyString emptyStringInstance;
+  return emptyStringInstance;
+};
 
 PROTOBUF_CONSTINIT std::atomic<bool> init_protobuf_defaults_state{false};
 static bool InitProtobufDefaultsImpl() {
-  ::new (static_cast<void*>(&fixed_address_empty_string.value)) std::string();
-  OnShutdownDestroyString(&fixed_address_empty_string.value);
+  ::new (static_cast<void*>(&fixed_address_empty_string().value)) std::string();
+  OnShutdownDestroyString(&fixed_address_empty_string().value);
 
   // Verify that we can indeed get the address during constant evaluation.
   PROTOBUF_CONSTINIT static const std::string& fixed_address_empty_string_test =

src/google/protobuf/message_lite.h

@@ -165,11 +165,10 @@ union EmptyString {
 
 // Default empty string object. Don't use this directly. Instead, call
 // GetEmptyString() to get the reference.
-PROTOBUF_EXPORT extern EmptyString fixed_address_empty_string;
-
+PROTOBUF_EXPORT extern const EmptyString& fixed_address_empty_string();
 
 PROTOBUF_EXPORT constexpr const std::string& GetEmptyStringAlreadyInited() {
-  return fixed_address_empty_string.value;
+  return fixed_address_empty_string().value;
 }
 
 PROTOBUF_EXPORT size_t StringSpaceUsedExcludingSelfLong(const std::string& str);