Optimize `Reflection::ScratchSpace::CopyFromCord()`.

If the Cord is already flat, return a pointer to its data instead of copying
them to a string.

PiperOrigin-RevId: 610434454

src/google/protobuf/message.h

@@ -101,6 +101,7 @@
 #include "absl/memory/memory.h"
 #include "absl/strings/cord.h"
 #include "absl/strings/string_view.h"
+#include "absl/types/optional.h"
 #include "google/protobuf/arena.h"
 #include "google/protobuf/descriptor.h"
 #include "google/protobuf/generated_message_reflection.h"
@@ -601,6 +602,9 @@ class PROTOBUF_EXPORT Reflection final {
     friend class Reflection;
 
     absl::string_view CopyFromCord(const absl::Cord& cord) {
+      if (absl::optional<absl::string_view> flat = cord.TryFlat()) {
+        return *flat;
+      }
       if (!buffer_) {
         buffer_ = absl::make_unique<std::string>();
       }