Add construct_array_builtin, deconstruct_array_builtin

There were many calls to construct_array() and deconstruct_array() for
built-in types, for example, when dealing with system catalog columns.
These all hardcoded the type attributes necessary to pass to these
functions.

To simplify this a bit, add construct_array_builtin(),
deconstruct_array_builtin() as wrappers that centralize this hardcoded
knowledge.  This simplifies many call sites and reduces the amount of
hardcoded stuff that is spread around.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/2914356f-9e5f-8c59-2995-5997fc48bcba%40enterprisedb.com

src/backend/utils/adt/name.c

@@ -314,11 +314,7 @@ current_schemas(PG_FUNCTION_ARGS)
 	}
 	list_free(search_path);
 
-	array = construct_array(names, i,
-							NAMEOID,
-							NAMEDATALEN,	/* sizeof(Name) */
-							false,	/* Name is not by-val */
-							TYPALIGN_CHAR); /* alignment of Name */
+	array = construct_array_builtin(names, i, NAMEOID);
 
 	PG_RETURN_POINTER(array);
 }