Re: SQLFunctionCache and generic plans

Since feature freeze is fast approaching, I did a tiny bit more
cosmetic work on this patchset and then pushed it.  (There's still
plenty of time for adjustments if you have further comments.)

I've discovered that starting from 0dca5d68d, the following query:
CREATE FUNCTION f(x anyelement) RETURNS anyarray AS '' LANGUAGE SQL;
SELECT f(0);

triggers:
TRAP: failed Assert("fcache->func->rettype == VOIDOID"), File: "functions.c", Line: 1737, PID: 3784779

On 0dca5d68d~1, it raises:
ERROR:  return type mismatch in function declared to return integer[]
DETAIL:  Function's final statement must be SELECT or INSERT/UPDATE/DELETE/MERGE RETURNING.
CONTEXT:  SQL function "f" during startup

Best regards,
Alexander Lakhin
Neon (https://neon.tech)