Fix SQL function execution to be safe with long-lived FmgrInfos.

fmgr_sql had been designed on the assumption that the FmgrInfo it's called
with has only query lifespan.  This is demonstrably unsafe in connection
with range types, as shown in bug #7881 from Andrew Gierth.  Fix things
so that we re-generate the function's cache data if the (sub)transaction
it was made in is no longer active.

Back-patch to 9.2.  This might be needed further back, but it's not clear
whether the case can realistically arise without range types, so for now
I'll desist from back-patching further.