Get constrrelid for fk constraints that lost it

I believe this should allow create constraint trigger
to look up the constrrelid out of the fk constraint
arguments for those cases where the FROM clause
was lost if we want to go that route.

It determines if it should look up args and which
argument to use based on the function name and won't
probably handle delimited names nicely. I figured
that since the point of it is to read old dumps,
this is okay. If the table doesn't exist or there
aren't enough args to intuit a table name, it currently
elog errors. Is that reasonable behavior, or should
it be taking the constraint and letting it fail at
runtime?