Simplify parse representation of savepoint commands

Instead of embedding the savepoint name in a list and then requiring
complex code to unpack it, just add another struct field to store it
directly.

Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>

src/include/nodes/parsenodes.h

@@ -2966,7 +2966,8 @@ typedef struct TransactionStmt
 {
 	NodeTag		type;
 	TransactionStmtKind kind;	/* see above */
-	List	   *options;		/* for BEGIN/START and savepoint commands */
+	List	   *options;		/* for BEGIN/START commands */
+	char	   *savepoint_name;	/* for savepoint commands */
 	char	   *gid;			/* for two-phase-commit related commands */
 } TransactionStmt;