Rename PLACEHOLDER code kind to WASM_FUNCTION.

src/snapshot/serialize.cc

Index: src/snapshot/serialize.cc
diff --git a/src/snapshot/serialize.cc b/src/snapshot/serialize.cc
index 2d52053294dbd6e2acd99359bdc3d87ca1dbdffb..0e60299f731950db53cd7a5ba871d370374cf063 100644
--- a/src/snapshot/serialize.cc
+++ b/src/snapshot/serialize.cc
@@ -2481,7 +2481,7 @@ void CodeSerializer::SerializeObject(HeapObject* obj, HowToCode how_to_code,
           SerializeGeneric(code_object, how_to_code, where_to_point);
         }
         return;
-      case Code::PLACEHOLDER:
+      case Code::WASM_FUNCTION:
         UNREACHABLE();
     }
     UNREACHABLE();