Implement LWG-3480, directory_iterator and recursive_directory_iterator are ranges

[fsb4000]

Fixes #2315

[CaseyCarter]

Hint to other reviewers: our end overloads for directory_iterator and recursive_directory_iterator already take their argument by value as in the proposed resolution of LWG-3480. In fact they always have, all the way back to MSVC-PR-108103! I'll have to thank Gor for realizing that WG21 would want to change these 3.5 years later =)

[No change requested.]

[StephanTLavavej]

No change requested, noting for the record: These explicit specializations work because the primary templates are defined in <xutility>:

STL/stl/inc/xutility

Lines 1972 to 1973 in 3c2fd04

	template <class>
	inline constexpr bool enable_borrowed_range = false;

STL/stl/inc/xutility

Lines 2696 to 2698 in 3c2fd04

	template <class _Ty>
	inline constexpr bool enable_view =
	derived_from<_Ty, view_base> || _Derived_from_specialization_of<_Ty, view_interface>;

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for range-ifying these iterators and getting the codebase closer to zero LWG issues! 😸 🚀 🎉