`<chrono>`: `[time.format]` may assume that strings are encoded in the active code page

[mnatsuhara]

We should investigate whether similar changes to #1834 are necessary for <chrono>'s integration with std::format.

[cpplearner]

A chrono conversion specifier must start with % (byte 0x25), and % isn't used as a non-lead-byte in any supported multibyte encoding (unlike { and }). So I think no changes are needed.