Move Cord copy constructor, copy assignment, and move assignment out of line.

Outlining these special member functions avoids duplicating inline tree reference counting and Cordz tracking across callers.

PiperOrigin-RevId: 1009904989

absl/strings/cord.cc

diff --git a/absl/strings/cord.cc b/absl/strings/cord.cc
index f9f576a..5d6727f 100644
--- a/absl/strings/cord.cc
+++ b/absl/strings/cord.cc
@@ -302,6 +302,12 @@
 // --------------------------------------------------------------------
 // Constructors and destructors
 
+Cord::Cord(const Cord&) = default;
+
+Cord& Cord::operator=(const Cord&) = default;
+
+Cord& Cord::operator=(Cord&&) = default;
+
 Cord::Cord(absl::string_view src, MethodIdentifier method) {
   const size_t n = src.size();
   if (n <= InlineRep::kMaxInline) {

absl/strings/cord.h

diff --git a/absl/strings/cord.h b/absl/strings/cord.h
index 8fe8e9f..fb3b86e 100644
--- a/absl/strings/cord.h
+++ b/absl/strings/cord.h
@@ -192,10 +192,10 @@
   // movable. The moved-from state is valid but unspecified.
   // Moves need to be declared since they are otherwise inhibited via the
   // declaration of the destructor.
-  Cord(const Cord&) = default;
+  Cord(const Cord&);
   Cord(Cord&&) = default;
-  Cord& operator=(const Cord&) = default;
-  Cord& operator=(Cord&&) = default;
+  Cord& operator=(const Cord&);
+  Cord& operator=(Cord&&);
 
   // Creates a Cord from a `src` string. This constructor is marked explicit to
   // prevent implicit Cord constructions from arguments convertible to an