Support string_view in caffe_importer

An upcoming change in Protobuf will change the return types of various
methods like Descriptor::name() and Message::GetTypeName() from const
std::string& or std::string to absl::string_view. This CL fixes users
of those methods to work both before and after the change.

modules/dnn/src/caffe/caffe_importer.cpp

@@ -126,8 +126,8 @@ public:
             const google::protobuf::UnknownField& field = unknownFields.field(i);
             CV_Assert(field.type() == google::protobuf::UnknownField::TYPE_GROUP);
             CV_CheckGE(field.group().field_count(), 2, "UnknownField should have at least 2 items: name and value");
-            std::string fieldName = field.group().field(0).length_delimited();
-            std::string fieldValue = field.group().field(1).length_delimited();
+            std::string fieldName(field.group().field(0).length_delimited());
+            std::string fieldValue(field.group().field(1).length_delimited());
             params.set(fieldName, fieldValue);
         }
     }
@@ -137,7 +137,7 @@ public:
         const Reflection *refl = msg.GetReflection();
         int type = field->cpp_type();
         bool isRepeated = field->is_repeated();
-        const std::string &name = field->name();
+        const std::string name(field->name());
 
         #define SET_UP_FILED(getter, arrayConstr, gtype)                                    \
             if (isRepeated) {                                                               \
@@ -189,7 +189,7 @@ public:
                 params.set(name, DictValue::arrayString(buf.begin(), size));
             }
             else {
-                params.set(name, refl->GetEnum(msg, field)->name());
+                params.set(name, std::string(refl->GetEnum(msg, field)->name()));
             }
             break;
         default:
@@ -212,7 +212,7 @@ public:
         {
             const FieldDescriptor *fd = msgDesc->field(fieldId);
 
-            if (!isInternal && !ends_with_param(fd->name()))
+            if (!isInternal && !ends_with_param(std::string(fd->name())))
                 continue;
 
             const google::protobuf::UnknownFieldSet& unknownFields = msgRefl->GetUnknownFields(msg);