One code path in addRangeTableEntryForFunction() neglected to assign
a collation to the tupdesc entry it constructs (which is a bit odd
considering the other path did do so).  This didn't matter before commit
5815696b, because nothing would look at the type data in this tupdesc;
but now it does.

While at it, make sure we assign the correct typmod as well.  Most
function expressions don't have a determinate typmod, but some do.

Per buildfarm, which showed failures in non-C collations, a case
I'd not thought to test for this patch :-(