Add dllexport_decl for generated default instance. (#13085)

Fixes #13084

Closes #13085

COPYBARA_INTEGRATE_REVIEW=#13085 from owent:add_dllexport_decl_for_generated_default_instance a46d4e8
PiperOrigin-RevId: 540938351

Author: owent

src/google/protobuf/compiler/cpp/file.cc

@@ -556,7 +556,7 @@ void FileGenerator::GenerateSourceDefaultInstance(int idx, io::Printer* p) {
             };
           };
 
-          PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT
+          PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT$ dllexport_decl$
               PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 const $type$ $name$;
         )cc");
   }
@@ -579,7 +579,7 @@ void FileGenerator::GenerateSourceDefaultInstance(int idx, io::Printer* p) {
           };
         };
 
-        PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT
+        PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT$ dllexport_decl$
             PROTOBUF_ATTRIBUTE_INIT_PRIORITY1 $type$ $name$;
       )cc");