Enable BEFORE row-level triggers for partitioned tables

... with the limitation that the tuple must remain in the same
partition.

Reviewed-by: Ashutosh Bapat
Discussion: https://postgr.es/m/20200227165158.GA2071@alvherre.pgsql

src/include/utils/reltrigger.h

@@ -29,6 +29,7 @@ typedef struct Trigger
 	int16		tgtype;
 	char		tgenabled;
 	bool		tgisinternal;
+	bool		tgisclone;
 	Oid			tgconstrrelid;
 	Oid			tgconstrindid;
 	Oid			tgconstraint;