WG14: Relax requirements on wchar_t to match existing practices

[tahonermann]

WG21 resolved issue #9 with the adoption of P2460R2 by removing the restriction that wchar_t be able to store all members of the wide literal character set (the restriction that wchar_t be able to store all members of the execution wide-character set remains though). This issue is intended to do likewise for C in WG14.

[ThePhD]

Forthcoming paper, combined with the new unicode functions, for C2y/C3a:

https://thephd.dev/_vendor/future_cxx/papers/C%20-%20Restartable%20and%20Non-Restartable%20Character%20Functions%20for%20Efficient%20Conversions.html

[tahonermann]

Thanks, JeanHeyd! Based on that, I'm assigning (well, more associating) this issue to you.