Add forward declaration for strings::CordReader and absl::Cord friendship

PiperOrigin-RevId: 933869588
Change-Id: I680d33f829443ee5672cb94f48e417676a1c8d84

absl/strings/cord.h

diff --git a/absl/strings/cord.h b/absl/strings/cord.h
index 22193b7..416b5ea 100644
--- a/absl/strings/cord.h
+++ b/absl/strings/cord.h
@@ -100,6 +100,10 @@
 #include "absl/types/optional.h"
 #include "absl/types/span.h"
 
+namespace strings {
+class CordReader;
+}  // namespace strings
+
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 class Cord;
@@ -858,6 +862,7 @@
   // public API call causing the cord to be created.
   explicit Cord(absl::string_view src, MethodIdentifier method);
 
+  friend class ::strings::CordReader;
   friend class CordTestPeer;
   friend bool operator==(const Cord& lhs, const Cord& rhs);
   friend bool operator==(const Cord& lhs, absl::string_view rhs);