pgsql: Make _SPI_execute_plan pass the query source string down to

Log Message:
-----------
Make _SPI_execute_plan pass the query source string down to ProcessUtility
if possible. I had left this undone in the first pass at the API change
for ProcessUtility, but forgot to revisit it after the plancache changes
made it possible to do it.

Modified Files:
--------------
pgsql/src/backend/executor:
spi.c (r1.174 -> r1.175)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c.diff?r1=1.174&r2=1.175)