Re: Add pg_basetype() function to obtain a DOMAIN base type

looking at it again.
I found out we can just simply do
`
Datum
pg_basetype(PG_FUNCTION_ARGS)
{
Oid oid;

oid = get_fn_expr_argtype(fcinfo->flinfo, 0);
PG_RETURN_OID(getBaseType(oid));
}
`

if the type is not a domain, work the same as pg_typeof.
if the type is domain, pg_typeof return as is, pg_basetype return the
base type.
so it only diverges when the argument type is a type of domain.

the doc:
<function>pg_basetype</function> ( <type>"any"</type> )
<returnvalue>regtype</returnvalue>
</para>
<para>
Returns the OID of the base type of a domain. If the argument
is not a type of domain,
return the OID of the data type of the argument just like <link
linkend="function-pg-typeof"><function>pg_typeof()</function></link>.
If there's a chain of domain dependencies, it will recurse
until finding the base type.
</para>

also, I think this way, we only do one syscache lookup.