P2447R6 Constructing `span<const T>` From `initializer_list<T>`

[StephanTLavavej]

WG21-P2447R6 Constructing span<const T> From initializer_list<T> LWG-4293 span::subspan/first/last chooses wrong constructor when T is const-qualified bool

Feature-test macro:

#define __cpp_lib_span_initializer_list 202311L

This ctor should participate in STL Hardening (#5090) like the other span ctors.

Note: We're focused on implementing the remaining library-only features in C++23. Until that's done, we will NOT be accepting PRs for C++26 features.

[lamarrr]

@StephanTLavavej I'd like to take on this if it's still available