FR347 27.12.10 Use "optional" in declaration of local_time_format instead of pointers

[wg21bot]

local_time_format should use optional<string> and optional<seconds> instead of pointers

Proposed change: change local_time_format signature to

local_time_format(local_time<Duration> time, optional<string> = {}, optional<seconds> offset_sec = {});

[HowardHinnant]

I'm ok with this change, but note that we can not do the analogous change for parsing. This is primarily due to the lack of support for optional<string&>, et al.

Both the formatting and parsing changes were explored with a partial implementation by Marshall, and in consultation with me.

[tituswinters]

LEWG in Belfast: Rejected. No consensus for change.