Cord: workaround a GCC 12.1 bug that triggers a spurious warning

See the GCC bug report
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105585

and Abseil bug report
https://github.com/abseil/abseil-cpp/issues/1175

Fixes #1175

PiperOrigin-RevId: 450083136
Change-Id: I207aaffaec9166b335065dd6ef148a721b94048e

absl/strings/internal/cord_internal.h

@@ -632,7 +632,9 @@ inline const CordRepExternal* CordRep::external() const {
 }
 
 inline CordRep* CordRep::Ref(CordRep* rep) {
-  assert(rep != nullptr);
+  // ABSL_ASSUME is a workaround for
+  // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105585
+  ABSL_ASSUME(rep != nullptr);
   rep->refcount.Increment();
   return rep;
 }