<filesystem>: `directory_entry::clear_cache` is non-standard

[cpplearner]

The directory_entry class has a member function clear_cache:

STL/stl/inc/filesystem

Lines 2178 to 2182 in c34f249

	#if _HAS_CXX20
	void clear_cache() noexcept {
	_Cached_data._Available = __std_fs_stats_flags::_None;
	}
	#endif // _HAS_CXX20

And this function is mentioned in <yvals_core.h> with paper number P????R?:

STL/stl/inc/yvals_core.h

Line 276 in c34f249

// P????R? directory_entry::clear_cache()

It appears that this function is part of the change to <filesystem> depicted in #232 (comment), but AFAIK the change did not materialize into a proposal.

Given that #232 was closed via #1343, is there still interest in this function?

[StephanTLavavej]

I am inclined to remove this non-Standard extension.

[StephanTLavavej]

We talked about this at the weekly maintainer meeting, and we agreed that this should be removed. @strega-nil-ms proposed that the removal should be outright, and if we hear user feedback, it could be restored via an opt-in macro, and this sounded great to everyone. We observe that this was never documented in the Microsoft Docs for directory_entry, so there should be virtually zero usage.