Re: Allow to_date() and to_timestamp() to accept localized names

I wrote:
> * I don't think it's appropriate to hard-wire DEFAULT_COLLATION_OID
> as the collation to do case-folding with. For to_date/to_timestamp,
> we have collatable text input so we can rely on the function's input
> collation instead, at the cost of having to pass down the collation
> OID through a few layers of subroutines :-(. For parse_datetime,
> I punted for now and let it use DEFAULT_COLLATION_OID, because that's
> currently only called by JSONB code that probably hasn't got a usable
> input collation anyway (since jsonb isn't considered collatable).

On closer look, it's probably a wise idea to change the signature
of parse_datetime() to include a collation argument, because that
function is new in v13 so there's no API-break argument against it.
It will never be cheaper to change it than today. So v11 below
does that, pushing the use of DEFAULT_COLLATION_OID into the
json-specific code. Perhaps somebody else would like to look at
whether there's something brighter for that code to do, but I still
suspect there isn't, so I didn't chase it further.

> Barring objections, this seems
> committable to me.

Going once, going twice ...

regards, tom lane