<chrono>: Formatting cleanups

[StephanTLavavej]

<chrono>

• Headers:
  • <chrono>: [time.duration.io] output #1341 included <sstream> unconditionally, but it's needed for C++20 only.
  • We need <iomanip> for put_time() in concepts mode only.
  • We don't need <iostream> (which provides the standard iostream objects cout etc.). Our implementation of <sstream> already drags in <istream> and <ostream>.
• We don't need to define _Is_hh_mm_ss; we can use _Is_specialization_v.
• Simplify _Choose_literal(); now that we have if constexpr, we don't need the struct layer.
  • Add _NODISCARD, top-level const, noexcept.
• When taking an unnamed _Auto_id_tag, there's no need for const.
• _Chrono_specs_setter::_Specs can be private; we haven't needed derived classes yet.
• In _Parse_chrono_format_specs(), we always return when _Begin == _End (i.e. at the beginning of the function, and whenever updating _Begin). Therefore, we don't need to check _Begin != _End when verifying that the chrono-specs begin with a conversion-spec.
  • "conversion-spec", singular, is the grammar term.
  • After previous refactorings, _Next_ch no longer has a purpose (and has a slightly confusing name now). We can simply switch on *_Begin.
• Add _STD qualification to format() calls.
• Upgrade the modifier table to static constexpr (no functional difference).
• _Chrono_formatter::_Write just needs a basic_ostringstream, instead of a general basic_stringstream.
• Undo /* strengthened */ rewrapping that had diverged from main.
• #undef _STATICALLY_WIDEN at the end of <chrono>, to avoid temptation.

P0355R7_calendars_and_time_zones_formatting/test.cpp

• This test should include <sstream>.
• stream_helper() just needs a basic_ostringstream.
• Mark print() as temporary, as it's unused.