Rename create_function_0 to create_function_c, and create_function_3
to create_function_sql, to establish their charters more clearly.
This should also reduce confusion versus our underscore-digit
convention for naming variant expected-files.
I separated this from the previous commit on the premise that keeping
the renaming distinct might make "git blame" tracking easier.
Discussion: https://postgr.es/m/1114748.1640383217@sss.pgh.pa.us
Tom Lane