Use struct-defined initialization when available

The  grpc_polling_entity and grpc_httpcli_response types have default
member initializer values specified, to indicate what a cleared variable
of those types should have as values.

This file overrides that, however, by directly performing a memset to 0
over the structures. Instead, the initialization values defined by the
type should be honored.

Local types that include these types are also upgraded to specify
default member initializer values to simplify clearing them.

This fixes -Wclass-memaccess warnings in modern versions of GCC.

test/core/util/port_server_client.cc

@@ -35,9 +35,9 @@
 #include "src/core/lib/http/httpcli.h"
 
 typedef struct freereq {
-  gpr_mu* mu;
+  gpr_mu* mu = nullptr;
-  grpc_polling_entity pops;
+  grpc_polling_entity pops = {};
-  int done;
+  int done = 0;
 } freereq;
 
 static void destroy_pops_and_shutdown(void* p, grpc_error* error) {
@@ -68,9 +68,9 @@ void grpc_free_port_using_server(int port) {
 
   grpc_init();
 
-  memset(&pr, 0, sizeof(pr));
+  pr = {};
   memset(&req, 0, sizeof(req));
-  memset(&rsp, 0, sizeof(rsp));
+  rsp = {};
 
   grpc_pollset* pollset =
       static_cast<grpc_pollset*>(gpr_zalloc(grpc_pollset_size()));
@@ -117,13 +117,13 @@ void grpc_free_port_using_server(int port) {
 }
 
 typedef struct portreq {
-  gpr_mu* mu;
+  gpr_mu* mu = nullptr;
-  grpc_polling_entity pops;
+  grpc_polling_entity pops = {};
-  int port;
+  int port = 0;
-  int retries;
+  int retries = 0;
-  char* server;
+  char* server = nullptr;
-  grpc_httpcli_context* ctx;
+  grpc_httpcli_context* ctx = nullptr;
-  grpc_httpcli_response response;
+  grpc_httpcli_response response = {};
 } portreq;
 
 static void got_port_from_server(void* arg, grpc_error* error) {
@@ -167,7 +167,7 @@ static void got_port_from_server(void* arg, grpc_error* error) {
     req.host = pr->server;
     req.http.path = const_cast<char*>("/get");
     grpc_http_response_destroy(&pr->response);
-    memset(&pr->response, 0, sizeof(pr->response));
+    pr->response = {};
     grpc_resource_quota* resource_quota =
         grpc_resource_quota_create("port_server_client/pick_retry");
     grpc_httpcli_get(pr->ctx, &pr->pops, resource_quota, &req,
@@ -202,7 +202,7 @@ int grpc_pick_port_using_server(void) {
   grpc_init();
   {
     grpc_core::ExecCtx exec_ctx;
-    memset(&pr, 0, sizeof(pr));
+    pr = {};
     memset(&req, 0, sizeof(req));
     grpc_pollset* pollset =
         static_cast<grpc_pollset*>(gpr_zalloc(grpc_pollset_size()));