Use C99 designated initializers for some structs

These are just a few particularly egregious cases that were hard to read
and write, and error prone because of many similar adjacent types.

Discussion: https://www.postgresql.org/message-id/flat/4c9f01be-9245-2148-b569-61a8562ef190%402ndquadrant.com

src/backend/access/transam/xact.c

@@ -199,27 +199,8 @@ typedef TransactionStateData *TransactionState;
  * transaction at all, or when in a top-level transaction.
  */
 static TransactionStateData TopTransactionStateData = {
-	0,							/* transaction id */
-	0,							/* subtransaction id */
-	NULL,						/* savepoint name */
-	0,							/* savepoint level */
-	TRANS_DEFAULT,				/* transaction state */
-	TBLOCK_DEFAULT,				/* transaction block state from the client
-								 * perspective */
-	0,							/* transaction nesting depth */
-	0,							/* GUC context nesting depth */
-	NULL,						/* cur transaction context */
-	NULL,						/* cur transaction resource owner */
-	NULL,						/* subcommitted child Xids */
-	0,							/* # of subcommitted child Xids */
-	0,							/* allocated size of childXids[] */
-	InvalidOid,					/* previous CurrentUserId setting */
-	0,							/* previous SecurityRestrictionContext */
-	false,						/* entry-time xact r/o state */
-	false,						/* startedInRecovery */
-	false,						/* didLogXid */
-	0,							/* parallelModeLevel */
-	NULL						/* link to parent state block */
+	.state = TRANS_DEFAULT,
+	.blockState = TBLOCK_DEFAULT,
 };
 
 /*