Down-integrate internal changes (#5467)

* Down-integrate internal changes

* Update generated code for php, objc and csharp.

* Add missing dependency in conformance_php
diff --git a/conformance/conformance_cpp.cc b/conformance/conformance_cpp.cc
index ac2f6de..ce333ee 100644
--- a/conformance/conformance_cpp.cc
+++ b/conformance/conformance_cpp.cc
@@ -137,6 +137,11 @@
     case ConformanceRequest::PAYLOAD_NOT_SET:
       GOOGLE_LOG(FATAL) << "Request didn't have payload.";
       break;
+
+    default:
+      GOOGLE_LOG(FATAL) << "unknown payload type: "
+                        << request.payload_case();
+      break;
   }
 
   switch (request.requested_output_format()) {