`<chrono>`: Investigate whether we can rework definition of `zoned_time` constructor

[mnatsuhara]

The Standard defines zoned_time's constructor with recursive constraints. We currently mirror the Standard's definition in our implementation, but we should investigate whether it's possibly to implement them non-recursively.

Examples:
zoned_time(string_view{})

STL/stl/inc/chrono

Lines 2980 to 2984 in 65eb507

	template <class _Traits2 = _Traits, enable_if_t<is_constructible< // TRANSITION, VSO-1303556
	zoned_time, decltype(_Traits2::locate_zone(string_view{}))>::value,
	int> = 0>
	// clang-format on
	explicit zoned_time(string_view _Name) : _Zone{_Traits::locate_zone(_Name)} {}

zoned_time(string_view, const sys_time))

STL/stl/inc/chrono

Lines 2993 to 2998 in 65eb507

	template <class _Traits2 = _Traits, enable_if_t<is_constructible< // TRANSITION, VSO-1303556
	zoned_time, decltype(_Traits2::locate_zone(string_view{})), const sys_time<_Duration>&>::value,
	int> = 0>
	// clang-format on
	zoned_time(string_view _Name, type_identity_t<const sys_time<_Duration>&> _Sys)
	: zoned_time{_Traits::locate_zone(_Name), _Sys} {}

[frederick-vs-ja]

These constructors are inconsistently constrained.

• For the default constructor, it's merely constrained on traits::default_zone() being well-formed, but there's no requirement that traits::default_zone() can be used to initialize TimeZonePtr.
• For the constructor from one string_view, the constraint says traits​::​locate_zone(string_view{}), but zone_ is then initialized with traits​::​locate_zone(name), where the value categories of the arguments of locate_zone are different.
• For other constructors that are "equivalent to" construction with {traits​::​locate_zone(name), ...}, the implicit constraints essentially require traits​::​locate_zone(name) to be convertible to TimeZonePtr.

Furtherly,

• in many constructors, it's unclear whether "initialize" means "direct-non-list-initialize". The differences between direct-initialization and direct-list-initialization are observable.

(Currently MSVC STL consistently uses direct-list-initialization.)

Edit: Oops, these functions can cause infinite meta-recursion if traits::locate_zone returns a string_view, and zoned_time(string_view) is really inconsistent.

Edit: Filed LWG-4067.