Prefer argument name over "$n" for the refname of a plpgsql argument.

If a function argument has a name, use that as the "refname" of the
PLpgSQL_datum representing the argument, instead of $n as before.
This allows better error messages in some cases.

Pavel Stehule, reviewed by Jeevan Chalke

Discussion: https://postgr.es/m/CAFj8pRB9GyU2U1Sb2ssgP26DZ_yq-FYDfpvUvGQ=k4R=yOPVjg@mail.gmail.com

src/test/regress/sql/plpgsql.sql

@@ -4811,3 +4811,12 @@ BEGIN
 END; $$ LANGUAGE plpgsql;
 
 SELECT * FROM list_partitioned_table() AS t;
+
+--
+-- Check argument name is used instead of $n in error message
+--
+CREATE FUNCTION fx(x WSlot) RETURNS void AS $$
+BEGIN
+  GET DIAGNOSTICS x = ROW_COUNT;
+  RETURN;
+END; $$ LANGUAGE plpgsql;