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/parser/parse_type.c

@@ -411,9 +411,7 @@ typenameTypeMod(ParseState *pstate, const TypeName *typeName, Type typ)
 		datums[n++] = CStringGetDatum(cstr);
 	}
 
-	/* hardwired knowledge about cstring's representation details here */
-	arrtypmod = construct_array(datums, n, CSTRINGOID,
-								-2, false, TYPALIGN_CHAR);
+	arrtypmod = construct_array_builtin(datums, n, CSTRINGOID);
 
 	/* arrange to report location if type's typmodin function fails */
 	setup_parser_errposition_callback(&pcbstate, pstate, typeName->location);

src/backend/parser/parse_utilcmd.c

@@ -1642,9 +1642,7 @@ generateClonedIndexStmt(RangeVar *heapRel, Relation source_idx,
 					elog(ERROR, "null conexclop for constraint %u",
 						 constraintId);
 
-				deconstruct_array(DatumGetArrayTypeP(datum),
-								  OIDOID, sizeof(Oid), true, TYPALIGN_INT,
-								  &elems, NULL, &nElems);
+				deconstruct_array_builtin(DatumGetArrayTypeP(datum), OIDOID, &elems, NULL, &nElems);
 
 				for (i = 0; i < nElems; i++)
 				{