[BUG] Accessor functions for std::array<T,0> aren't constexpr

[sunnlok]

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Description

Accessor functions for std::array<T,0> aren't constexpr.
This means that std::array can not be used in a constexpr context if its, for example, used to initialize a span, which might or might not be empty depending on other compile time evaluated factors.

Current implementation:
_LIBCPP_INLINE_VISIBILITY value_type* data() _NOEXCEPT {return reinterpret_cast<value_type*>(__elems_);} _LIBCPP_INLINE_VISIBILITY const value_type* data() const _NOEXCEPT {return reinterpret_cast<const value_type*>(__elems_);}

Fixed in LLV since version 11:

https://github.com/llvm/llvm-project/blob/release/11.x/libcxx/include/array

[DanAlbert]

Duplicate of #1530