Mark new text<->date, text<->time, text<->timetz conversion functions as

noncachable, so that CURRENT_DATE and CURRENT_TIME work as functions
again, rather than being collapsed to constants immediately.  Marking the
reverse conversions noncachable might be overkill, but I'm not sure;
do these datatypes have the notion of a CURRENT value?  Better safe than
sorry, for now.