jit: Reference function pointer types via llvmjit_types.c.
It is error prone (see
5da871bfa1b) and verbose to manually create function
types. Add a helper that can reference a function pointer type via
llvmjit_types.c and and convert existing instances of manual creation.
Author: Andres Freund <andres@anarazel.de>
Reviewed-By: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/
20201207212142.wz5tnbk2jsaqzogb@alap3.anarazel.de