Add locale getter in tm_writer

[SappyJoy]

Why not?

Example of usage in custom DateTime writer:

template <typename OutputIt, typename Char>
class DateTimeWriter : public fmt::detail::tm_writer<OutputIt, Char>
{
    using super = fmt::detail::tm_writer<OutputIt, Char>;

public:
    DateTimeWriter(const std::locale & loc,
                   OutputIt out,
                   const std::tm & tm,
                   const int32_t usec,
                   const char * zone)
        : super(loc, out, tm)
        , m_usec(usec)
        , m_zone(zone)
    {
    }

    void on_tz_name()
    {
        super::out() = detail::write_tm_str<Char>(
                super::out(), m_zone, super::locale());    // <----- tm_writer::locale()
    }

[vitaut]

Please apply clang-format.

[vitaut]

Merged, thanks.